Originally Posted by
DrIsotope
where the total force accelerating the object downwards is the difference between the force of gravity and the drag force; in short, heavier things fall faster. The weight is (usually) enough to overcome the relative differential in drag-- that is, a lighter rider on a lighter bike produces more drag relative to the force gravity is exerting on them. In the real world, my rider + bike weight is about 50lbs greater than my wife's; unless she drafts me down a hill, I will just pull away from her the entire way down.
You can also notice this if you go snow skiing with a significant other or friend that is a lot smaller than you are. Again, there are a lot of factors in play (ski length is a big one), but generally speaking a bigger skier will get pulled down the slop just that little bit faster.
And if you really want to see some long debates about the physics of gravity, object mass and wind resistance, check out some of the WW2 airplane forums where they argue this stuff ad nauseam.
[MENTION=152773]noglider[/MENTION], on the few occasions that I get to chug along on a flat stretch without interruption, I have actually noticed that my heavier bike seems to maintain speed a bit easier (in terms of my perception) than my lighter bikes, especially if it is windy. Once I get the momentum up on the heavier bike, which definitely takes more effort, it just seems to plow along with less sensitivity to outside forces.