I have been trying to work this out from basic principles but the answers
are at best foggy:

Consider two electric motors (commutator) with identical inputs, say 250W,
and identical RPM, say 15,000. What are the factors that determine the motor
efficiency and thus the output power?

If I understand the physics, the output power is directly related to torque
if rpm are held constant. What then, in the nature of the motor
construction, will increase its torque? Is the diameter of the rotor one of
the factors?

Thanks,

--
Michael Koblic,
Campbell River, BC