alt Hacker NewsYep, you're getting it. Same power, same efficiency, same power dissipation, smaller motor, smaller dissipative area, higher temperature.
The other assumption I probably should have stated is that the two motors are made of similar materials with similar temperature limits. We know the ambient temperature and we know the maximum temperature of the materials used. So for a component made of those materials, existing in that ambient temperature, with an additional heat load proportional to the waste heat in the motor...
The ability to shed heat (assuming similar forced fan cooling, as we were) determines the amount of power we can deliver to the device without increasing its temperature.
So, ok, under a whole bunch of stated and unstated unproven assumptions, a smaller motor of the same power delivery as a larger motor is more efficient. There's no relation to reality here. I don't even know why I thought the idea in your comment that started this thread was worth pursuing.