[David Cooper]

If you have a short fat cyclist and tall thin one of the same weight, there will be no difference from the tyres as they have the same weight pressing on them (ignoring aerodynamic lift if the shapes of the riders generate different amounts - you'd have to select ones with the same component of lift). The tall thin cyclist should win out every time (unless you add other aspects of shaping to slow the thin one down).

In normal situations out on the road, of course, the fat riders are heavier than the thin ones, and they are faster downhill. On a CTC run, I was once pedalling flat out down a steep hill on my road racing bike and a freewheeling woman overtook me. (She wasn't actually fat, but I was at that time close to the minimum healthy weight for a racing cyclist of average height who specialised in climbing, so she didn't have to be fat to be faster.)

[Divij Gala]

Are there any mathematical calculations that could be used to prove what you have stated in your article?

[Rob]

I am an engineer.   Sosjay is correct.  Air friction will not slow the heavier ride as rapidly as the light rider.  Both have the same acceleration due to gravity, g, but the more massive person has a greater force F = mass X g.  This greater force is significant in overcoming frictional resistance (counter force) from air.    Aerodynamics can help reduce friction but mass will always trump aerodynamics on down hill slopes where air resistance persists (notwithstanding terminal velocity etc.).