it has to do with the interplay between aero drag and gravity

the heavy rider has approximately the same frontal area as the lighter rider (actually the heavier rider usually does have a little more frontal area), so the force due to aerodynamic friction (drag) is approximately equal. However, the horizontal component of gravity with respect to the road surface (the part of gravity that pulls you forward) is greater with the heavier rider than with the lighter rider - this is assuming that both riders are applying the same amount of power to the pedals.

To put it simply:

10 - 2 = 8 is greater than 8 - 2 = 6

In order to match the heavier rider on the descent, the lighter rider would need to apply enough additional power at the pedals to overcome the deficit between their two gravitational forces.