Vacuum (lift) is how wings works, it's how airplanes fly. It's how airboat props make thrust. My rudders are 4' tall by 42" long. That is 2,016 in^2 of rudder area. Ambient air pressure at sea level is 14.7 PSI. If I draw just 0.1 PSI of vacuum on average on the back side of my prop I net 2016 * 0.1 = 201.6 lbs of force.
Now look at where that force is applied. With smaller rudders the turning force is closer to the transom. The larger the rudder, the further stern of the transom the rudder applies it's force. That is the leverage part. It's the same reason these rudders suck in a cross wind.