I alluded to the reason that I think is most commonly cited. I'm not sure how much hard evidence there is, but generally it's the size. When it gets over a certain size, then the ability of all the members to really participate in governance is diluted, and that opens up the opportunity for corruption, for things to happen in secrecy, for money to be absconded with, and so on.
On April 30th, 2012. See this statement in context.