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?
The main thing? Optimization. Mobile games are built for mobile, so naturally, graphical stuff like polygon-count, particle effects, texture resolutions, shadow quality, etc. are all toned down to be able to run smoothly on mobile hardware.
Couple that with the vastly smaller screen sizes and the diminishing returns of graphical power to visuals (e.g. visual jump from low shadows to medium is further than medium to high), and you’re getting a fantastic mobile gaming experience for a tiny fraction of the power consumption.