Originally Posted by dgregory57
The extra energy a large climber exerts in effect stores more kinetic energy that is used on the descent. Think of it as a gravity "spring"... a large rider has a stronger spring.
Sort of yes, but the bulky ones do need more force to make them accelerate equally fast. So the two cancel each other out in the end.
As has already been said, in a vacuum, the skinny and the clydesdale would descend at a similar pace. However, while the fat guy has, say about 1 1/5 the weight of the skinny, he only has 1 1/4 his frontal surface, i.e. drag. Just like a cube that has twice as long sides as a smaller cube will have 2x as big side surfaces and 4x as big weight.
So the bigger rider has proportionally less drag to fight on the downhill (This is assuming they don't pedal much.)