Originally Posted by
cooker
Basically, the gravitational force is proportional to the weight of the rider while air resistance is proportional to cross sectional area. A rider who is twice as heavy as another rider - say 220 lbs vs 110 lbs, is not usually anywhere near twice as big in cross section. Weight increases roughly in proportion to the cube of height or waist circumferance, while cross section increases roughly in proportion to the square of height or waist circumferance. So in downhill coasting, weight pwns aerodynamics.
This is why heavier riders are faster on downhills. And it's the same reason that sprinters can be big and strong. Their extra power for a slightly larger aerodynamic resistance makes them faster in a sprint than lightweight riders.