From my semi-understanding of pinball history, Harry Williams figured out pretty quickly the drunks in depression-era Chicago taverns were going to savage his machines to get the balls to go to the desired destination, and invented a tilt bob.
Once the game moved onto the floor with flippers, some bulk likely had to be added to keep them stable and durable enough to still withstand being pounded on and shaken around. That'd be my guess. If they could be smaller, that would have been worked out by now.

No kidding, a routine double-danger move on a WPC would scoot a modern Stern half a foot across the floor.