In really simple terms, the smaller diameter has less drag, so will tend to spin faster. The race boat trick cookee is tlking about is basically to get more speed, by using a smaller diameter, you load the engine back up by upping the pitch (Pitch is like the gears on your car, higher gear = high load). Race boat speed is slightly important, so the higher pitc, the faster you go!
So, if youi drop the diameter, at the same pitch the small dia prop will spin faster.
Other thing is that a smaller prop has less"grip" o nthe water, so will tend to "let go" more easily (e.g. corners, rapisdly opened throttles, waves) which with race boats isn't such a big problem as they are generally lighter than a cruising rib where "all round grip" is more important......
I certainly wouldn't run different props on twins. If the prop was new, can you not P/X it back in for the original size?