Basically, it seems to me like the technology in mobile GPUs is crazier than desktop/laptop GPUs.

It's not. They have the same software technologies and the desktop counterparts have better hardware.

but not by enough that it seems to need to be 100x bigger than a mobile GPU.

Yes it is. No benchmarks would agree with you here. Also, just look at the power draw for each and how much noise each cooling solution makes.

And top end mobile GPUs actually perform quite admirably when it comes to graphics and power.

Depends entirely on what you see as admirable. Power efficiency wise, they're great, but their performance isn't anything to write home about, especially considering that they typically share cooling solutions with the CPU. And that's at the top of the line. Lower down, it's not all that great, with desktop counterparts having much better 1% lows when the power is more comparable.

So with most of what you said being incorrect, your conclusion is also incorrect. Generally, more surface area on coolers means they can cool higher power limits, can have bigger fans and/or have those fans spin slower so they're much quieter. Regarding the power consumption, it's simply diminishing returns. Mobile GPUs are just cut sooner on the graph.