user7788: I know that killing virtuous beings is a grave bad karma. But when it comes animals, virtue is irrelevant. I have heard from monks that bigger the animal greater the sin it is to kill it. How is that so? Why does physicality matter at all? Isn't it just a matter of your intention when it comes to karma? 

user254: In the novel The Dalai Lama's Cat, the Dalai Lama is quoted as teachiing,

I understand what you're saying, but there are some who would say that eating a cow is better, because a single cow can provide more than one thousand meals. A fish, only one meal. Sometimes it takes many prawns, many sentient beings, for only one meal.

And,

It's a very big subject. You will find that some people tell you there is only one way, this way, which happens to be the way they think, and that everyone else should change their views to be like them. But it is really a matter of personal choice. The important thing is to make sure our decisions are guided with compassion and wisdom.

This is a subject on which different people (and, maybe, different schools of Buddhism) differ.
You wrote, "But when it comes animals, virtue is irrelevant" -- I don't know whether there's any Buddhist school/tradition which that's an accurate description of.
The kind of opinion I quoted above is not based on "virtue is irrelevant", instead it's based on "life is dear to all".

user23808: Buddhaghosa, an influential 5th-century Theravada commentator, states:

With regard to animals it is worse to kill large ones than small. Because a more extensive effort is involved. Even where the effort is the same, the difference in substance must be considered. In the case of humans the killing is the more blameworthy the more virtuous they are. Apart from that, the extent of the offence is proportionate to the intensity of the wish to kill.

So it seems to be the additional effort that makes it morally worse to kill bigger animals. Effort is a direct expression of cetana (volition). As Peter Harvey states:

cetana is the motive for which an action is done, its immediate intention (directed at a specific objective, as part of fulfilling a motive), and the immediate mental impulse which sets it going and sustains it.

References:

E. Conze, Buddhist Scriptures, Page 70, Penguin, 1976.
P. Harvey, An Introduction to Buddhist Ethics: Foundations, Values and Issues,
Page 17, Cambridge University Press, 2012.


user471: According to the Bhikkhu Patimokkha (quoted below), which are rules for monks, killing a human is grounds for immediate and irreversible dismissal from the monastic order (parajika).
However, deliberately killing an animal is an offense that requires only confession (pacittiya) within the monastic order, usually with the intention not to do it again. It's not as severe as killing a human.

Intentionally bringing about the death of a human being, even if it is
still a fetus — whether by killing the person, arranging for an
assassin to kill the person, inciting the person to die, or describing
the advantages of death — is a pārājika offense. (Pr 3)
Pouring water that one knows to contain living beings — or having it
poured — on grass or clay is a pācittiya offense. Pouring anything
that would kill the beings into such water — or having it poured — is
also a pācittiya offense. (Pc 20)
Deliberately killing an animal — or having it killed — is a pācittiya
offense. (Pc 61)
Using water, or getting others to use it, knowing that it contains
living beings that will die from that use, is a pācittiya offense. (Pc
62)

According to AN 6.87, there is actually a list of people a person could kill or injure, and such a person will never be able to have the right mental state or moral capacity to learn the Dhamma:

"Endowed with these six qualities, a person is incapable of alighting
on the lawfulness, the rightness of skillful mental qualities even
when listening to the true Dhamma. Which six?
"He has killed his mother; he has killed his father; he has killed an
arahant; he has, with corrupt intent, caused the blood of a Tathagata
to flow; he has caused a split in the Sangha; or he is a person of
dull discernment, slow & dull-witted.
"Endowed with these six qualities, a person is capable of alighting on
the lawfulness, the rightness of skillful mental qualities even while
listening to the true Dhamma. Which six?
"He has not killed his mother; he has not killed his father; he has
not killed an arahant; he has not, with corrupt intent, caused the
blood of a Tathagata to flow; he has not caused a split in the Sangha;
and he is a discerning person, not slow or dull-witted.

According to MN 86, Angulimala killed many human beings, but he could still change and become an arahant. So, probably, he did not commit the transgressions above (killing father, killing mother, killing arahant, injuring Buddha)
So this shows the value of humans above animals, and the value of the Buddha, the arahant and one's parents above all other humans.
So, the answer is yes, there is a difference in the consequence and effects of killing.
With this, we can extrapolate, that it's worse to kill a dog or cow, than a mosquito or cockroach. It's not the physical size, but it's the mental and emotional capacity of the animal.
That said, it would still be impossible for an arahant to kill any animal, however small, as the arahant is free of all mental defilements. In other words, to do any killing at all, you must have mental defilements.

user295: The intensity of Karma in killing of a being including an animal can be influenced by these 2 factors:

intensity of volition - more planning or effort the worse (elephant vs ant), more stronger your desire to kill the worst
conciseness of the being - if the being has a higher level of conciseness (elephant vs ant) and virtue (bear vs cow) the worst. Hence virtue of the animal also plays a role. E.g. sakes are to have more wroth than more tame animals.


user7792: My knowledge in Buddhism is quite poor. IMO it is a greater sin to kill a larger animal than to kill a smaller animal but this cannot be the case always! Let me ask you a question. Which is easier, to kill an ant or to kill an elephant? Generally the effort and planning you have to go through to kill an elephant is much greater than to kill an ant. The greater the effort you are willing to put is because of the greater intensity of cruelness of your mind which makes it a greater sin. True physicality isn't what determines this. Hope this answers the question 

7788: <p>I know that killing virtuous beings is a grave bad karma. But when it comes animals, virtue is irrelevant. I have heard from monks that bigger the animal greater the sin it is to kill it. How is that so? Why does physicality matter at all? Isn't it just a matter of your intention when it comes to karma? </p>
 254: <p>In the novel <em>The Dalai Lama's Cat</em>, the Dalai Lama is quoted as teachiing,</p>

<blockquote>
  <p>I understand what you're saying, but there are some who would say that eating a cow is better, because a single cow can provide more than one thousand meals. A fish, only one meal. Sometimes it takes many prawns, many sentient beings, for only one meal.</p>
</blockquote>

<p>And,</p>

<blockquote>
  <p>It's a very big subject. You will find that some people tell you there is only one way, <em>this</em> way, which happens to be the way they think, and that everyone else should change their views to be like them. But it is really a matter of personal choice. The important thing is to make sure our decisions are guided with compassion <em>and</em> wisdom.</p>
</blockquote>

<p>This is a subject on which different people (and, maybe, different schools of Buddhism) differ.</p>

<p>You wrote, "But when it comes animals, virtue is irrelevant" -- I don't know whether there's any Buddhist school/tradition which that's an accurate description of.</p>

<p>The kind of opinion I quoted above is not based on "virtue is irrelevant", instead it's based on "life is dear to all".</p>
 23808: <p>Buddhaghosa, an influential 5th-century Theravada commentator, states:</p>
<blockquote>
<p>With regard to animals it is worse to kill large ones than small. Because a more extensive effort is involved. Even where the effort is the same, the difference in substance must be considered. In the case of humans the killing is the more blameworthy the more virtuous they are. Apart from that, the extent of the offence is proportionate to the intensity of the wish to kill.</p>
</blockquote>
<p>So it seems to be the additional effort that makes it morally worse to kill bigger animals. Effort is a direct expression of <em>cetana</em> (volition). As Peter Harvey states:</p>
<blockquote>
<p><em>cetana</em> is the motive for which an action is done, its immediate intention (directed at a specific objective, as part of fulfilling a motive), and the immediate mental impulse which sets it going and sustains it.</p>
</blockquote>
<p><strong>References:</strong></p>
<ol>
<li>E. Conze, Buddhist Scriptures, Page 70, Penguin, 1976.</li>
<li>P. Harvey, An Introduction to Buddhist Ethics: Foundations, Values and Issues,
Page 17, Cambridge University Press, 2012.</li>
</ol>
 471: <p>According to the <a href="https://www.accesstoinsight.org/tipitaka/vin/sv/bhikkhu-pati-intro.html" rel="nofollow noreferrer">Bhikkhu Patimokkha</a> (quoted below), which are rules for monks, killing a human is grounds for immediate and irreversible dismissal from the monastic order (parajika).</p>
<p>However, deliberately killing an animal is an offense that requires only confession (pacittiya) within the monastic order, usually with the intention not to do it again. It's not as severe as killing a human.</p>
<blockquote>
<p>Intentionally bringing about the death of a human being, even if it is
still a fetus — whether by killing the person, arranging for an
assassin to kill the person, inciting the person to die, or describing
the advantages of death — is a pārājika offense. (Pr 3)</p>
<p>Pouring water that one knows to contain living beings — or having it
poured — on grass or clay is a pācittiya offense. Pouring anything
that would kill the beings into such water — or having it poured — is
also a pācittiya offense. (Pc 20)</p>
<p>Deliberately killing an animal — or having it killed — is a pācittiya
offense. (Pc 61)</p>
<p>Using water, or getting others to use it, knowing that it contains
living beings that will die from that use, is a pācittiya offense. (Pc
62)</p>
</blockquote>
<p>According to <a href="https://www.accesstoinsight.org/tipitaka/an/an06/an06.087.than.html" rel="nofollow noreferrer">AN 6.87</a>, there is actually a list of people a person could kill or injure, and such a person will never be able to have the right mental state or moral capacity to learn the Dhamma:</p>
<blockquote>
<p>&quot;Endowed with these six qualities, a person is incapable of alighting
on the lawfulness, the rightness of skillful mental qualities even
when listening to the true Dhamma. Which six?</p>
<p>&quot;He has killed his mother; he has killed his father; he has killed an
arahant; he has, with corrupt intent, caused the blood of a Tathagata
to flow; he has caused a split in the Sangha; or he is a person of
dull discernment, slow &amp; dull-witted.</p>
<p>&quot;Endowed with these six qualities, a person is capable of alighting on
the lawfulness, the rightness of skillful mental qualities even while
listening to the true Dhamma. Which six?</p>
<p>&quot;He has not killed his mother; he has not killed his father; he has
not killed an arahant; he has not, with corrupt intent, caused the
blood of a Tathagata to flow; he has not caused a split in the Sangha;
and he is a discerning person, not slow or dull-witted.</p>
</blockquote>
<p>According to <a href="https://www.accesstoinsight.org/tipitaka/mn/mn.086.than.html" rel="nofollow noreferrer">MN 86</a>, Angulimala killed many human beings, but he could still change and become an arahant. So, probably, he did not commit the transgressions above (killing father, killing mother, killing arahant, injuring Buddha)</p>
<p>So this shows the value of humans above animals, and the value of the Buddha, the arahant and one's parents above all other humans.</p>
<p>So, the answer is yes, there is a difference in the consequence and effects of killing.</p>
<p>With this, we can extrapolate, that it's worse to kill a dog or cow, than a mosquito or cockroach. It's not the physical size, but it's the mental and emotional capacity of the animal.</p>
<p>That said, it would still be impossible for an arahant to kill any animal, however small, as the arahant is free of all mental defilements. In other words, to do any killing at all, you must have mental defilements.</p>
 295: <p>The intensity of Karma in killing of a being including an animal can be influenced by these 2 factors:</p>

<ul>
<li>intensity of volition - more planning or effort the worse (elephant vs ant), more stronger your desire to kill the worst</li>
<li>conciseness of the being - if the being has a higher level of conciseness (elephant vs ant) and virtue (bear vs cow) the worst. Hence virtue of the animal also plays a role. E.g. sakes are to have more wroth than more tame animals.</li>
</ul>
 7792: <p>My knowledge in Buddhism is quite poor. IMO it is a greater sin to kill a larger animal than to kill a smaller animal but this cannot be the case always! Let me ask you a question. Which is easier, to kill an ant or to kill an elephant? Generally the effort and planning you have to go through to kill an elephant is much greater than to kill an ant. The greater the effort you are willing to put is because of the greater intensity of cruelness of your mind which makes it a greater sin. True physicality isn't what determines this. Hope this answers the question </p>
