This is a good question. Is the answer that a larger sail has a greater "Moment of Inertia" for the same lift as a smaller sail. ie the centre of area (effort, lift) is higher up. This moment (forcexdistance) is counteracted by the moments of board rail and fin. "moment of inertia" isn't the perfect phrase to use but my engineering studies were a while back now. I think its also called "2nd moment of area". The bigger sail "moment" is balanced by the fin. With a smaller sail the lateral forces and forward speed generated will be similar but with less moment generated. For the same speed in the water the fin is generating the same lift, it just that the sail isn't balanced against it. Don't think beer has helped me explain myself here. bye.