It is the will of the Force, youngiing.
Seriously though: I think it's because
a)Big avatar=big loading time for slower connections
b)what the hell's the point?
c)Big avatars don't look as good

TRP/GSGDante/2SQD/1PLT/1COM/1BAT/1RGT/VEA/VE*HeavyWeapons *Read the bloody manual!"Never believe a rumour of my demise. I have as many lives as a cat. Also as many teeth, as many claws, and the same cheery, cooperative disposition." Peter Wiggin,
XenocideWashington:"I knew this plan would never work!"
Church:"None of our plans ever work."
Caboose:"That's why we carry guns."