They do get awfully hot if you're walking around. I'm pretty sure that the foot ones are made for low-oxygen environments and walking around keeps the air circulating and they get really hot. Obviously, your feet are also less cold when you're walking so it's a double whammy.
I have rather poor circulation in my feet and hands so they don't get too hot for me. My dad can't stand to use them on his feet either, says they get too hot.
I wish hunting season was late spring and early fall.