...and another thing:

when the prop turns, some of the power from the engine is converted into thrust, which pulls the engine forward, and some energy is lost. High rpms and small props "waste" more power than large props at low rpms.

So even if two engines put out the exact same amount of power, but at different rpms, the lower revving engine will produce more USABLE power because more of its output is converted to thrust. This is one of the reasons why most park flyers are geared. The engine puts out the same power, but a large prop gives more thrust per horsepower than a small one.