Was just messin' 
My understanding is that torque is measured as force x distance. foot-pound, oz-inch, kg-cm, Nm are examples of units you will often see.
The torque of the motor is a constant at a constant voltage / current, so as the distance increases, the force is reduced.
eg: 20 kg-cm at 1 cm affords 20kg of force. At 2 cm, you're going to max out at 10kg. Assuming constant current.
So the smaller wheels should afford you more force at the circumference of the wheel, but the torque is the same.