Your iMac is not producing any more heat per unit of computational work than your other machines unless it is significantly older and thereby less efficient. The iMac doesn't conduct the heat away from the processors and into the room as quickly as a larger desktop would, so the chips operate at higher temperatures under load. This does not have any effect on the actual amount of energy dissipated into the room over the long run.
That makes sense to me and all... but the difference is quite noticeable. As in, I need to take off some layers if I am doing something on my imac, but not with the others.
If I had to guess, it probably has mostly to do with the imac sitting closer to me while typing, and also that I tend towards using it for more graphics heavy tasks (since it has the largest screen). Also, I am in the southeast, so heat changes are unusually noticeable, especially in the summer.
You are right though, the imac probably is as efficient or even more efficient than my other machines, however it is more noticeable when the heat is in your face instead of blowing out off next to a baseboard.
