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.