Even without wind resistance factored in, the heavier rider has an advantage going downhill. The extra weight increases momentum, which helps overcome rolling resistance. Imagine two bowling balls atop a 30-degree slope, say 100 yards long. The red ball weighs 20 pounds and the blue one weights 1 pound. Start them both rolling with a gentle tap and you'll see the blue one gain a slight initial lead ... then the red one will pick up mo' and finish far ahead. Also, big riders go faster on flats not because of differences in weight but because they generally have more power. If a 140-pound rider and a 300-pound rider each put out an average of 300 watts on a long, flat section of road the 140-pound rider will go faster, again based on having less rolling resistance to overcome (and probably less wind resistance as well).