Is it necessary to have big paper during PhD? I have a present PhD fellow from another research group who gets pressured by his PI to produce a "big paper" before he could graduate. He has already data and paper ready for publication but his PI continues to add more work and scope to make it "big".  Is it really necessary to produce big work at this level? What is the cons and pros of publishing big and small paper?
In every place that I know of, and likewise in every field that I know of, you have to please your advisor in order to complete your degree. I don't know of any situation in which it can be done otherwise short of litigation. 

In some places the advisor signs the dissertation. In some s/he signs a form indicating the candidate has successfully completed the degree. There can be variations, such as assigning a grade. 

So, the answer to the question here can be yes or no, but that depends on the individual advisor, so in the case at hand, it seems to be yes. Having a good relationship to your advisor is essential. Perhaps "big" can be negotiated, but the advisor needs to be sufficiently happy. 



I should also note that in some fields, publishing before completion of the degree is the norm. In other fields it isn't expected and can be quite rare.