[eon]

Contrary to what seems obvious, the contact patch of your tires has no impact on the amount of grip you have. All that matters is the coefficient of friction between the two surfaces and the amount of weight (or pressure) pressing them together. If you halve the surface area you double the pressure and the amount of grip remains the same. However halving the surface area increases the amount of load the rubber has to endure and heat to dissipate, so I suspect that’s why powerful bikes have fat rear tires (leaving aside aesthetics in some cases).

http://en.allexperts.com/q/Physics-1358/2009/1/surface-area-effect-friction.htm