every time you go up in diameter or more blades you increases "traction" in the water. This puts more load on your engine. It can increase efficiency and change handling properties. every time you increase your pitch, the propeller is trying to go further (distance) in the water. Like a screw through wood. It takes more power to do this. Your engine has a limited amount of power to give you. It is always a good idea to keep some of it in the back pocket and not squeeze every hp out of it; for longevity of your expensive engine.