If they are both Retina, then the weight difference is the biggest deal. Reducing the screen size from 13" to 12" would let them make it significantly lighter than the MacBook Pro.
Just because a smallish form factor proved propular doesn't mean you're not suddenly going to find a size which is a dealbreaker.
12" is tiny and doesn't really improve portability for a laptop-like device (which goes in a backpack).