A magnet assembly alone is not a motor as was stated by James Bang. You need a coil and a magnet to have a motor. To oversimplify it, the coil determines the thermal power handling and the soft parts determine the mechanical power handling. Other than wicking away heat, due to the mass of metal being near the coil and having provisions to force air over the coil, the magnet assembly has very little to do with the question your asking. It's more to do with the total mass of aluminum or copper involved.