Q:

Massive power jump at levels corresponding to expansions

When I hit level 61, I noticed a massive jump in power; Something like 10 times the damage. I saw this immediately, without any gear change; I just started one-shoting everything. It looks like this is limited to the old world. Some time later, I got equivalent super powers against enemies in Outland. Research on the internet implies that this is related to a "stat squish" that took place in the 6.x/Warlords of Draenor timeline.
How can I predict when and if I will get these super powers, and against which enemies? Is there any documentation on this?

A:

There are two different ways that a player could experience this type of rapid power increase.
Level Difference Scaling Mechanics
This is what you specifically encountered.
As you touched on there was a large "stat-squish" that took place with the launch of the Warlords expansion that made stat increases between expansions much more linear and much less exponential. One of the potential problems that this could have introduced was making older content harder even when significantly out levelling that content. So to prevent this issue they also introduced an algorithm that would both increase your damage output and also reduce damage taken when you are a higher level; this effect also grows as the level gap between you and the content increases.
In this scenario you can expect the "super powers" you experience to show up any time you are more than a level or two above what you are fighting.
Explosive ILvl Increase
At the end of each expansion there was extensive "End Game" content where you would keep getting better gear (same level requirement but higher iLvl). So at the time when the next expansion was released the baseline for the new gear would have an iLvl somewhere around the mid to high level range of the previous end game gear.
Now when you level a new character you are skipping all of that end game gear improvement so when you start into content for the next expansion you will see much higher iLvl increases when replacing pieces.
For example look at this quest from Ice Crown which offers iLvl 174 rewards
and compare it to this quest from Mount Hyjal which offer iLvl 272 rewards.
Where someone who took part in the ICC raid would have had between iLvl 251(10 man normal) and 277 (25 man heroic).
Note: The power jumps come from how iLvl relates to the amount of stats on an item and is a topic too broad for this answer but the tl;dr; is higher iLvl = more stats per iLvl
In this scenario you can expect to see those "super powers" generally after you've earned a few new pieces from the new content and last up until the creature level catches up.
Here are the level ranges for this: 

Vanilla -> Burning Crusade (level 58-60)
Burning Cruside -> Wrath (level 68-70)
Wrath -> Cataclysm (level 80)
Cataclysm -> Pandaria (level 85)
Pandaria -> Warlords (level 90)
Warlords -> Legion (level 98*-100)

*I'm pretty sure they re added the 2 level grace with legion

