Amplifiers vary in efficiency. This depends on their design. The less efficient, the excess power needs to be vented as heat. It depends too on stuff like the case design, the K5 Pro I think has a quite big (and particularly tall) case most of which is empty. I know with some other stuff, like Topping has one small amp, the A30 Pro, which does a very large amount of power but in a very small case, and it reportedly gets very hot, hotter than something like the A90 which has similar power but in a larger case. The A30 needs ventilation holes all over which the A90 doesn't. The dongle I have that gets hot, the MoonRiver 2, is also particularly thin for the power level.