Silent_Buddha
Legend
You need more volume to dissipate the heat and/or faster spinning fans, but that doesn't forcibly translate to "thicker".
To maintain battery life it'd have to be thicker. There's no two ways around it.
The "tablet" portion only contains what, 30-40% of the total battery capacity of this convertible. You want as low a power consumption as possible while having a CPU that gives you decent performance. Putting the GPU in the dock with the rest of the batteries makes sense if you're optimizing the weight of the tablet portion.
Move up to a 35W CPU and suddenly you're going to need significantly more battery (weight) as well as significantly more cooling (noise and bulk) which inevitably leads to a thicker, heavier, and noisier device.
Which then means your keyboard dock needs to be significantly heavier in order to not tip over easily every time you attach the tablet portion. And suddenly you find your device is likely 50-100% heavier, 50-100% thicker, and far noisier than the device you had before.
Ultrabooks with long battery life (which is basically what the Surface Book is) aren't gaming machines. There's a reason for that. And the Surface Book faces far more design challenges than an average Ultrabook.
Regards,
SB