As far as I know, the Guild size can be purchased by in game gold. It's kind of a sign of how rich your team are.
Yesterday, I went through the Guild list. Here is what I found out.
1 place:
FUN: 50/50
2 place:
RK: 42/44
3 place:
FK: 34/36
I think there are some teams may didn't show up in my Guild list.
If you like, you can post you team size, or even some team extra-boost in this thread.
Edit:
Just got a splash screen.