Actually, all other things being equal, changing the voltage to a motor will not change the size of the motor. It will change the size of the conductors needed to feed the motor, and it will change the requirements for the switchgear needed to control the motor. It will change the termination requirements inside the motor, and it will change the insulation requirements. But once you account for these differences, the motor size will stay the same for different voltages.
When you increase the voltage to a motor, the current required to deliver the power goes down, and thus the required wire size goes down. But since you have the same rotor spinning with the same magnetic flux going through the same winding cross section, the volts/turn stays the same, and thus you needs a greater number of turns. The result is that the higher voltage version of the motor has more turns of thinner wire, with the same 'net slot current' (current per conductor times number of conductors in the slot).
As with building wire, the same insulation system is pretty much used for everything below 600V, so the insulation thickness for a 120V motor and a 600V motor will be essentially the same. At higher voltages, special insulation systems are required; the insulation starts taking up a lot of space at this point and will change the size of the motor. I've seen a white paper describing motors built with the same sort of insulation systems used for underground high voltage cables; the idea is that you run your motor directly at 69KV, and eliminate the need for a step down transformer. The motor itself gets larger, to make room for the extra insulation.
If you really want to make a motor small, what you need is _speed_. The size of a motor pretty much scales with _torque_, not power. Rotational mechanical power is the product of torque and speed (just as electrical power is the product of volts time amps). So you can double the power of a motor by doubling its speed. 400Hz motors are _very_ small for their power rating. The limit here is the mechanical capability of the various parts to deal with the rotational stresses.
-Jon
