former x86-64 hardware manufacturer view: the max TDP in watts of a CPU that can reasonably fit in something as thin as an air is a lot lower than a macbook pro or similar cooling solution (combined heatsink/heatpipe/blower fan).
in the intel world there's a whole class of CPUs that absolutely max out at 8W, 12W or 15W, you can go somewhat higher in a fatter laptop.
If I had to guess I'd say that the air probably thermal throttles or reaches a steady-state of lower performance at an earlier time than the Pro, if the pro has a larger cooling package on the same CPU...
If you were to sit the two side by side on a desk and run a complicated pure-cpu x265 1.5 hour ffmpeg encode of a 4K video from raw y4m, the pro will probably race ahead after about 20 minutes, after the heat has soaked into the air.
Yes, that's exactly how it works. I'm just not sure why you're saying this. It's obvious, given the fact that both machines use the same chip and different cooling.
It has nothing to do with the shape and size of the board, which differs between the two machines significantly.
If it's exactly the same board and layout, an ultra slim laptop motherboard will be designed for different thermal limits than a desktop board. Also a laptop board will have its shape and orientation optimized for a very thin heatpipe + flat blower cooling system, something in a desktop might be different. Take for instance the same CPU used in a gaming laptop or in an Intel NUC/tiny box shaped PC, the heatsink/fan arrangement will be unique between the two. But then again this new imac is so thin it might as well be a laptop.
