Talking about free-falling bodies at all is WAY off track. Unless you miss a turn.

We are talking about rolling resistance on an inclined plane plus (or actually minus) wind resistance. Rolling resistance on an inclined plane is a vector equation that takes into account three forces, acceleration due to gravity, normal force (perpendicular to the plane) and rolling resistance. The rolling resistance portion takes mass into account. So this favors the heavier rider. All of that is just for a straight flat surface at an angle.

Then, momentum comes into effect getting through turns and over small bumps. Momentum is directly proportional to mass, so this favors the heavier rider.

The wind resistance part is a parasitic loss. You could almost take that out for low speeds. Or if both riders can get the same aero tuck and have the same frontal area, regardless of weight. Chances are that the heavier rider has a larger frontal area, so this favors the smaller rider, but this will be a very small differance in a tuck. This is also a smaller portion of the equation until you get to very high speeds.

Gearing can make a differance. A 53-11 of course will get you going faster than a 52-12 before you have to let nature take over. It will also determine if you can accelerate out of a turn.

Skills, skills, skills. Braking, accelerating, and "the line". Read a performance driving or motorcycle book for some insight. If you have to learn about the line, you will never really be great at it, but you can improve.

I am a bigger rider (195lbs) and I blow by people going down. Going up, I just blow.

So, small guys, gain your time going up and increase your skills going down.

Light weight is a bigger advantage over all. Here is why. Lets say you and I are doing a 5 mile 1000 ft climb and then coming back down. If you are 2mph faster going up and I am 2mph faster going down, you will win. The reason for this is that there is more time spent going up, so your average speed for the 10 miles is higher.