I think several people are partially right in this discussion. Your weight does not affect your speed attained due to the force of gravity going down hill. Velocity = initial velocity + acceleration * time. Acceleration due to gravity is a constant here on Earth at 9.8 meters/s^2. There is no room for mass in this equation.

Now your acceleration do to other forces will be affected by your mass. The higher the mass the more difficult it will be to acheive the same acceleration. Force = mass * acceleration. So acceleration = force / mass. Apply the same force and the lighter
person will go faster.

Also the higher the mass the greater the momentum and the harder it is to slow you down. If a heavy person and a light person apply the same pressure to the brakes the light person is going to stop faster. The same wind is going to slow the lighter person more.

Given all of that I don't know why some people are faster down hills than others. I read something somewhere (maybe here or on Sheldon Brown's page) that said some people are just faster down hill because they are. It could be they don't hesitate at the top so their initial speed is faster. They may have a better Aero position. They may hit the brakes less. They just may not have the fear of going fast. The only thing I do know is it is not acceleration due to gravity. It might be mass and momentum so other forces slow them less but it is not gravity.

Steve