My reasons? Firstly, to more clearly be able to see the lovely photos of women the users here have up as their avatars. Secondly, I feel that 150 x 250 px is a more reasonable size in this age where so many people have very high-res monitors (and I'd be willing to bet that lots of porn watchers have very high-res monitors). Nobody wants to have to get out their microscope to be able to more clearly see an avatar.
Anyway, thanks.