BL motors and load

Ok, so this just crossed my mind. Do brushless motors really run that much less efficient with a lesser load (IE- being "undergeared" in a truck)? As evidence, I always read posts about people being undergeared on other forums, and see that their motors are running obscenely hot.

The reason I ask this is because I just took the plunge and ordered that 4800kv V2 4 pole motor, and I remember back when people were running pletty motors (which some were also 4 poles, as you know) on Uforces, they'd have to use some crazy pinions to keep the motor temperature in check.

But anyways, my original question; how come such a load is needed on the motor in order for it to operate more efficiently? Does it have to do with it having to expend energy one way or another, as in, if X amount of watts goes in the motor, X-Y must come out of the motor (Y being heat, X being work done to move the gears and wheels and stuff). So unloaded, you have more Y than X due to conservation of energy (same amount of energy goes in, but some is burned off as heat, and since you're undergeared in this scenario, not near as much wattage is going to be needed to perform work, so that large amount of wattage leftover is heat).

If anyone has any links or explanations, please post them. I'm curious. This'll give me a good starting point for gearing the V2 motor as well.