It's not so much about limited space but more about as technology evolves, and things get smaller and thinner
I'm positive a GTX970 runs cooler than a GTX280, and a 970 is a lot more complicated + a hell of a lot more powerful than a 280. So complexity does come into it but not so much.
Well if you've been paying attention you'll know that's not true. Just for fun try comparing a GTX 1080 to a GT 630.
Components get smaller (transistors, etc.) But as the whole gets more complicated it gets bigger and less efficient, which requires more space and cooling (a 630 doesn't even have a cooler). That's why full towers exist.
The 280 uses 60% more power (max). Of course it'll run hotter.
The GTX 200 family is also particularly hot. Grab the 550 and put it against a 1080ti. There is a considerable power consumption increase (150W to 250W). We could spend all day pulling examples from the Internet. Pointless.
Just think about it. The GT 630 used a 40nm process, and had 97 CUDA cores. The 1080ti uses a 16nm process, and has 3500 CUDA cores. I'll leave the math to you. It's fair to assume complexity means more than cientific breakthroughs when it comes to size.
A simple GT-630 even fanless can be just as cool in an ITX/MATX situation-case housing as long as the cooling is sufficient. It does not need a huge anvil full ATX case with 10 fans to keep it cool
On the contrary. As the die shrinks, less power is required and the chip runs colder.
I would love to have my next build be a miniITX gaming powerhouse.