Using imperial, Torque= (HPx5252) / RPM
Therefore, changing the RPM mechanically, while not changing the power output, changes the torque.
For example, 1HP motor@ 1000rpm = (1x5252) / 1000 = 5.252ftlb
If you used pulleys to reduce the speed to 500RPM, it would be (1x5252) / 500 = 10.504ftlb
True, if all you do is put a bigger wheel on the end of the shaft, you're just putting excess strain on the mechanics of the motor and accomplishing nothing. The RPM has not changed.
However, if you take that quarter inch shaft, put a 2" wheel on it, then use that to run a belt which wraps around a 4" wheel, you will decrease the resulting rpm by half, because it takes two full turns of the original shaft to turn the second one once. This slower moving second wheel has twice the torque of the second. And yes, this is a factor of diameter, i.e.it is a lever.