The fact that they turn the same prop at the same speed on the ground doesn't give you the whole picture. The FX quite possibly gets more HP at a higher RPM so it may do better with a smaller prop, also it may turn the same prop at a higer speed in the air. That would give it the same thrust if hovering, but better verticle at speed, and higher speed straight and level. Now you may not need the extra speed but someone else may.