Originally Posted by
xenologer
In other words, while the guy twice as big recieves twice as much force from gravity, he also has twice as much mass which cancels out the benefit.
This is true, but you have to consider all the forces acting on the rider. The net force is the gravitational force minus the force of air resistance. Air resistance increases as speed increase. At some point, the gravitational force is canceled out by the force of air resistance, at which point the rider will cease to accelerate and maintain a constant speed (assuming incline, wind, etc. remain constant). This is called terminal velocity. A heavier rider has a higher terminal velocity.