Bandwidth costs, and nothing sucks it down faster than large animated avatars & sig pictures!
I'm against both. I like avatars as they make it easier for me to keep people straight, ('cept for "Steal-yer-mates-avatar Days" over at Rage3D where we all swap around avatars for a couple of days just to confuse people...it REALLY confuses people!), and find my own posts so I can know where I left off in a thread....but increasing the size of 'em is just a bad plan.
They're good at what they are, but don't try and make too big a deal out of them or they'll distract from what your words are.
(Yeah, says the man who made his avatar in a fit of inspiration at 2am one night and who has come to be recognized by it...