i see some people with bigger avatars that have less post count than me? Or is it based on trade count?
It's not based on anything as far as I know. However, the larger avatars may be hosted somewhere else, and the forums don't check the size of those, only the ones that are uploaded.
Avatar size has nothing to do with anything, except the size that you choose to make it. Don't make it too big or I'll kick you in the balls.