Assuming similar aerodynamics (a bad assumption, but go with it for a second), a heavier rider will have an easier time overcoming air resistance because he will have greater momentum. Think of dropping a baseball vs. dropping a wiffle ball. So that's what gives the heavier rider an advantage.

Of course, a heavier rider is likely to present a larger cross-section than a lighter rider, so some of this advantage will be lost but not all. Put the heavier rider on a recumbent and it's no contest, even with the extra air resistance from his beard.