Most likely that constraint is the shape of the battery. They probably did the same thing they do in the unibody mac, which is make the battery fill every possi

Most likely that constraint is the shape of the battery. They probably did the same thing they do in the unibody mac, which is make the battery fill every possible nook and cranny, with no real compartmentalization to speak of. With that type of design, you can get more capacity and a thinner device, but user service would probably be so difficult that people would be breaking things left and right. And even though users would be breaking their own devices, they would be frustrated and take it out on Apple, tarnishing their feel-good image. A $99 replacement program is much more beneficial PR-wise.