In perfect conditions (IE-Vacuum) then weight means nothing. Gravity acts the same on all bodies.
But in the real world, other things must be considered. Primarily, aerodynamics. It's possible for the heavier rider to have better sectional density than the lighter one. It has been observed that clydesdale-sized riders do tend to gain on their lighter brethren on steep descents.
But there's a point of vanishing returns, however. If you are so large as to present a surface area that overcomes the sectional density advantage...
Also, if the descent has a lot of turns, the lighter rider will be at advantage again, as he'll be able to drive harder through the turns without loosing traction.