said by Oedipus:
Nvidia/AMD also bundle software (ie, Catalyst Control Center) but their driver downloads are packaged to work with a wide variety of cards. I would imagine that adds to the bulk as well.
NVIDIA drivers are the size they are because they need to be that size. The only thing that arguably isn't a driver, despite it's uses is the control panel, which is 15-20MB. The next big thing is the 64/32bit stuff, that's about 15MB or so of almost identical files. The biggest portion of the driver package by far goes to PhysX, which is 50MB+ due to the way PhysX was implemented by AGEIA, which requires all the old versions to be included for them to continue working in those games.
This is all compressed mind you, so the actual installed size of that ~170MB package is probably around 250MB. Other than a few basic display drivers (25MB) that run at all times, most of it actually isn't being used at any given time.