QUOTE: since the light guy loses more of his momentum to air resistance on the way down, he won't coast uphill as far. END QUOTE
uh, wouldn't the big guy have a greater gravitational force slowing him down up the hill? (sorry if this was stupid)
Yes, but the resulting acceleration (deceleration, if you prefer) would be the same as for the light rider. Gravity pulls harder on the big guy, but his mass resists it, and they balance out.
Think of trying to pull a standing freight train up to speed. If there are 50 cars, and it takes one locomotive 5 minutes to accelerate it to 40 miles an hour, then if there were 100 cars, it would take two locomotives 5 minutes to accelerate it to 40 miles an hour. Twice the mass, twice the force, same acceleration. Gravity is like that. It pulls harder on a heavier mass than on a light one, but the acceleration is the same.