From a purely seat-of-my-pants-im-trying-to-remember-college point of view, I believe the heat is caused far more by pumping tons of electricity, rather than physical size/layout.

A big problem (and also probably why they're so thin) is that at a certain size latency becomes a huge issue. This is where clock speed increases come into play (electrons move faster) along with die decreases (electrons can travel shorter distances).

If you can run 100mph, but you have to run 200 miles vs. someone who can walk 1mph, but only has to walk 10 feet... I wish I had some more concrete examples but I can't come find anything off the top of my head.