The difference is the diameter of the wheel and the corresponding hole in the tire. 1.9 is smaller, so a 1.9 tire has a smaller opening in the middle. 1.9 and 2.2 wheels/tires do not interchange well because the tire with the wrong wheel will be too loose or too tight. 1.9 wheels/tires tend to be narrower, as well.

1.9 wheels and tires look more 'scale' in size and will tend to roll under the body while 2.2 wheels/tires usually stick out and generally do not fit the wheel well of the vehicle. 2.2 is more like a modified truck with a lift kit.