Hello, I too wonderd about this a while back. Someone then enlightened me by explaining that the propeller is not moving the aircraft by the blowing of air back, but it is more drilling its way into the air. So, the fact that you may only have 2 in of a 10 ft dia prop..(an exageration) extending beyond the cowl, the prop is still screwing its way into the air and pulling the aircraft with it.
Dan