This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
> I was wondering if anyone could help me figure out why heavier
> people roll down hills faster than the little scrawnies like myself.
Surface as well as cross sectional area of an object (a human body)
increases more slowly than its weight (volume). Therefore, wind drag,
that is largely dependent on surface, is proportionally smaller for a
heavier and larger object than a smaller one of similar shape and
composition. A good example is dust at a rock quarry that remains
suspended in the air for a long time while the larger pieces such as
sand, gravel, and rock fall increasingly faster to the ground. They
are all the same material and have similar irregular shapes but have
different weight to surface area ratios, and therefore, different wind
resistance to weight ratios. This applies equally to bicyclists
coasting down hills if other factors such as clothing and position on
the bicycle are similar.