To answer this question you first must assume that the golfer in this example had a perfect golf swing and is always on plane and is flawless on every shot. If that golfer had a grip that was too small, his hands would over release and rotate, which would cause the ball to draw (right to left on a right handed golfer). If that golfer had a grip that was too big, his hands would under release and rotate, which would cause the ball to fade (left to right on a right handed golfer).