You are confusing effective with efficiency the smaller higher pitched prop is always more efficient, howeve unless you have a sleek fast pland and want to go as fast as possible it is not more effective. Also the total efficiency of the system must be considered. Even though a small high pitched prop is more efficient, it will make a plane go faster which means more drag, this may mean more fuel burned for a given distance which is less efficient. But we as modelers are not concerned about that. So my recommendation is that untill you are working on full scale aircraft think more in terms of being more effective not more efficient.