From what I can tell, Super Mario Galaxy has more stages, but you don't spend as much time in each stage. I would have preferred the stages to be a little bigger and to have more stars in each stage, a la Mario 64. Still the fact that they added more controls and made so many stages in the first place gives it major points.
Instead of having larger stages, they just have lots of tiny little planets in one stage, so it seems like the stage is smaller.