This is soooo far from technical and accurate but just thinking about it, it seems intuitive. A blade of grass can wiggle/oscillate much faster than a heavy steel vault door, smaller= more nimble and quick in most cases. I'll stick with that theory.
Besides reducing power and increase speed, smaller transistor also allow the obvious of putting more transistor on a given size chip The smaller the chip the cheaper they are to make, so it's desirable to get as many transistor on a chip as possible.