Bigger sensor means bigger lenses means thicker device.
But on another idea, I don't know anyone that carries around a separate camera anymore. I've laughingly watched people use iPads as cameras, which is pretty hilarious. Getting images off of a portable camera has always been a PITA, requiring dongles, cables, incantations. It would be cool if Apple enabled AirDrop to be used on portable cameras. When it works, AirDrop just rocks. No dongles, cables, incantations; point and click, so to speak...
The camera companies all seem to have their ways of massaging image data inside their devices. Making images easier to transfer to Apple devices would be fantastic, and then Apple software can 'Appleize' the images. *shrug*