I'm happy to answer any questions around this! We've been working on this since update since we launched the product last year, so we're excited to be able to share it today.
I wondered this too, but they will need a way to be able to plug the same Mainboard into a new chassis such that the Expansion Cards work correctly, or else design a new Mainboard with increased size (or I guess longer Expansion Cards, but that seems silly).
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.