Part of the reason it's so big is because you can play games on the screen as well.
For example, if you're playing on your TV and there's a show on TV you want to watch while you play or your wife/girlfriend/kids (assuming most here are adults) want to watch TV, you can just flip the game into the controller itself with the press of a button and play on the screen inside the controller while someone watches TV.
I agree that the controller is generally pretty large, but from what I hear in articles from people who have played/tested it out, it's a lot more comfortable than it looks and is extremely light.
I'm in the position of "won't judge until I get to play with it", but generally I like where it's going/is at right now. At least it's not focused on motion controls.