That has nothing to do with air drag. The heavier cars win because they weigh more and have greater inertia, so they go farther before stopping on the flat part of the track below the ramp.
It's not that simple. We don't live in a vacuum. Since we're in 6th grade, go to your nearest cub scout pine wood derby race and try to win with an un weighted car. I ride with little racer boys and easily smoke them on descents. By your understanding of physics, greater wind resistance, greater rolling resistance etc., they should drop me like a rock on the descents just like they do on the climbs! Man, that'd suck!!!