Think of a gear in a car (a real car). Lower gears the motor runs at a higher RPM but goes slower, and has much more pulling power. Shift to a higher gear and the car goes faster but the engine RPM is lower and the power is less. You accellerate faster in first gear than you will in, say third gear, but you go faster in third gear than in first.

Same thing with props. Lower pitch means more pulling power but less speed and higher RPM. Higher pitch means faster speed at lower RPM and thus less power. The engine has only so much power to turn a prop and adding a bigger ie. higher diameter higher pitch puts more demand on the engine and it only has so much power do it will not turn as fast as with a smaller prop.

Decide what you want out of the engine prop combination and go from there. If you want good vertical performance, go with a lower pitch for more pulling power, but beware of the RPM and noise level.

DS.