Horsepower is basically Torque x RPM, Turning an 11 x 10 at, say, 17,000 RPM requires more horsepower than turning a 6 x 10 at 17,000 RPM, because the larger prop needs more torque than the smaller prop. When an airplane is flying along level at a steady speed, thrust is equal to drag. I'm sure folks more expert than me will elaborate.