Nope, you are taking identical riders and bikes. You state the example, but your math is for identical riders and bikes. If not, then you would have to put these variables in the equations. You have not done that. You are taking weight as surface area, that is kind of ok, but again, you are taking identical riders and bikes. Just because the total surface area of the riders are roughly calculated, you are not taking into account how the bikes and riders are aerodynamically. I agree that the drag will most likely be more important than the inertia, unless the drag is the same. Just because two things have the same weight and surface area does not mean they are the same aerodynamically.
The real problem I had with your original statement is:
Originally Posted by john bono
If rider positions and clothing are the same, the only reason why the smaller rider out accelerates the larger is rolling resistance.
This is totally false, unless you are talking about identical bikes and riders that have the same aerodynamics. Otherwise all the other factors come into play on how fast a bike will roll down a hill, not just rolling resistance.
The original poster wanted to know why the other rider was slower, there are many reasons why this could be. You tried to make it a simple argument about tire rolling resistance by neglecting other factors.