I saw in -sir's customization thread that the biggest avatar that you can buy is the 150x200 one. So where this huge avatar comes from? I don't know his size, but i would say a minimum of 150x300.
If you check his avatar now though, it's not as big as in the picture.
Perhaps he put the avatar too big and later got warned for it?
Maybe the pic is under 500 kb?
His avvy got resized so it was probably edited to fit the requirements.