Of course (?) they need to build a larger motherboard. I'd be really tempted to buy a 15" model especially with a touchpad with 3 hardware buttons (I'm not considering any laptop without them) and a keyboard without numberpad (very important but a little less than the hw buttons.)
I can imagine chassis designs that use the same Mainboard but move the Expansion Card slots out. It would also be much more in keeping with the ethos of the product and company to use exactly the same Mainboard in every device.
