AFAIK mobile SOCs aren't thermally constrained at the moment.
Heat dissipation is just not much of an issue from the SOC.
Why did all the tablets get dual core processors months before any phones did? Why can't intel just toss an ivy bridge CPU into a phone?
They most certainly are thermally constrained, in that the thermal limitations determine the types of processors that are even considered.
This shouldn't be under debate, it's simple physics. Same energy consumption in a smaller package = hotter. Given most of the example phones, the "small" phone is really only 3-5% smaller, so the difference is insignificant and maybe not even speed bin worthy, but that doesn't change the laws of physics.
If you can build a given phone a given size with given specs, making the exact same thing smaller will give you a hotter device.
Look at Desktop CPU, look at large laptop mobile CPU, look at ultrabook CPU, look at netbook CPU, compare to high end tablet CPU, and finally look at tiny phone processors. Smaller devices support slower CPU, things work that way for a reason, it isn't arbitrary.
The screen actually generates heat in a smartphone not dissipates it.
Can't it do both? I'm fairly certain a screen dissipates heat, unless it's coated with some sort of inward reflective heat coating that causes it to hold all heat inside the phone. Nope, it is not.