Simply put, the bigger avatars are available to those who contribute to the site. Doesn't make them any more special than the non-contributing user, it's just a perk. I have no problems with banning a contributor to the site if they act up, same as I do a non-contributor..
Not real sure I see what all the fuss is, but then again, I maintain a neutral outlook on damn near everything.
Also, the ranks are based on posts as well. I don't remember what posts equal what level, as that's never been a big concern of mine.
If you have any questions or issues, don't hesitate to PM myself, Trevelyn1015, or Mnemonic. Cheers!