Organizing people is a difficult problem which only gets more difficult as youmore people need to be organized.
The larger a company is the more of its internal structures, rules, policies, history, etc are devoted just to organizing people.
For me, realizing this was like the first time you hear a flourescent light buzzing in an otherwise quiet room.
Reasonable people can differ on this point, but for my own sake I'd much rather avoid all the people-organizing baggage that comes with large companies.
I don't have a hard-and-fast rule about the size of a place I want to work but the larger a place is then generally the more reason I need to want to be there.
Of course, this is all kind of theoretical at this point, as Flipstone is my forever home.