You are making one big mistake. Two items of dissimilar weights will fall at the same acceleration
in a vacuum. However, aerodynamic drag is a function of surface area (n^2), while mass is determined by volume(n^3). So let's look at this situation:
Scenario: I'm on my cyclocross bike with 700x28c slick tires. The bike weighs 20lbs with all the doodads I put on it. I weigh 180lbs. My friend is on his GT mountain bike which weighs 26 lbs and is equipped with 26x1'' (25c) slicks. He weighs 215lbs. When starting to roll down hill at the same speed I continually accelerate ahead of him although neither of us is pedaling and there's no aerodynamic advantage. Our tires are inflated to within 5 psi of each other. He has a 41 lb advantage. WHAT IS GOING ON?
First, let's look at the forces of acceleration:
Rider 1:180 lbs + 20 lb bike = 200 lbs total
Rider 2:215 lbs + 26 lb bike = 241 lbs total.
So the ratio of acclerative forces due to gravity 1:1.205 from rider1:rider2. "So what?!?" you say, "Rider 2 is heavier than rider 1, so those forces even out!" But that is if you don't take aerodynamics into account. I'm making an assumption here, but to determine the surface area of the riders, let's just get the cube root of the total weight, and square it, which should give us an appoximation of surface area, and therefore drag(no, this isn't a perfect ratio, but I don't have a wind tunnel to do it properly).
Rider 1: 34.1
Rider 2: 38.7
The ratio of forces due to drag is 1.13. In order for the 200lb rider+bike to out accelerate the 241lb rider+bike, he needs to reduce his overall drag by 13% relative to the larger rider in order to make up the difference. If rider positions and clothing are the same, the only reason why the smaller rider out accelerates the larger is rolling resistance.