The proprietary drivers are definitely smaller on Linux (
~33mb on disk on my system) but that's largely because the Windows drivers contain a shitload of what are essentially runtime-patches for games. A lot of stuff, particularly shaders, in games is poorly coded and nvidia essentially rewrite them, stick them in the drivers, then replace them in the game when it runs. Everyone wins - the games run better and nvidia cards look good. These take up a *lot* of space.
They don't bother porting that stuff to Linux.