For anyone who knows.
Basically, it seems to me like the technology in mobile GPUs is crazier than desktop/laptop GPUs. Desktop GPUs obviously can do things better graphically, but not by enough that it seems to need to be 100x bigger than a mobile GPU. And top end mobile GPUs actually perform quite admirably when it comes to graphics and power.
So, considering that, why are desktop GPUs so huge and power hungry in comparison to mobile GPUs?
They are actually not that much bigger or different from mobile or game console GPUs, they just have a lot of cooling bolted to them. The cooling allows them to sacrifice efficiency, to be more power hungry and more powerful.