Comparing actual physical chip size, a desktop GPU isn't 100x bigger than a mobile GPU, more in the range of 10x. What you're used to seeing is the large PCB to handle more I/O, plus the heat sink, fans, and plastic shroud. The heat sink is needed because, at the end of the day, a desktop GPU might be pulling 300W+ of power and that energy has to go somewhere. A phone GPU on the other hand is likely to max out somewhere around 5W of power, and a standard laptop might be around 15-30W, neither of which need nearly the surface area to dissipate the heat.

why are desktop GPUs so huge and power hungry in comparison to mobile GPUs?

Put simply, they're doing more calculations per unit of time. According to wikipedia, an Adreno 750 (high end phone GPU) is pushing ~5 TFLOPS (FP32), while an RTX4090 can push 82.58 TFLOPS (FP32). That's 82.58 / 5 = 16.516 times more operations per second. 16x the performance for 10x the chip size and ~100x the power. (Estimating cost is kinda difficult, but a 4090 is $1600 msrp, while according to this article the cost of a Snapdragon 8 gen 3 which has the Adreno as part of its SoC is ~$200. So the price of just the graphics is probably worth at least half that. So the cost is also ~16x, which means relatively similar FLOPS per dollar, before accounting for power usage).

If your question is "how does 100x the power justify 16x the performance?", think of it like a 90hp economy car vs a 1000hp sports car. If you are ok with accelerating 0 to 60 over the course of a minute, you can do that very efficiently and minimize your gas usage. But if you need to go 0 to 60 in <3s, there's only one way that's going to happen, and that's absolutely DUMPING energy into that engine as fast as possible. It's going to generate a lot of wasted heat, it's going to get awful gas mileage, but it will go as fast as mechanically possible (with the engine technology we currently have). And that's what a 4090 is doing. It might not be the best performance per watt, but if you need the performance it's simply your only option.

If your question is actually "why do mobile games look so good relative to the best looking high end AAA games?", that's called good art direction. With proper optimizations and shortcuts that make assumptions about time of day, camera angles, distance to objects, resolution, etc, you can render a pretty decent looking scene these days. But where it usually falls apart is dynamic lighting, because that requires more calculations per pixel. Notice you won't see many moving light sources, shadow casting, transitioning between times of day, or advanced materials in mobile games. What you do see was carefully and deliberately placed where you are most likely to notice, and shortcuts were taken in ways that you hopefully won't ever question it.

Since the the dawn of computer rendering, all of gaming, from low power to high, is about taking shortcuts to make as good looking of a scene as you can with the hardware you've got. And we've gotten pretty good at doing that, to the point that it's relatively difficult these days for the untrained eye to spot the difference.