[Jeff in Kentucky]

Skinny tires equals less weight and more speed, if the engine power and weight are low enough to not break the tire traction in corners.

Here is a good article about 125cc 2-stroke MotoGP bikes, soon to be 450cc 4-strokes instead like a dirt bike race engine:

http://www.fasterandfaster.net/2007/06/ktm-vs-suzuki-125cc-gp-racer-vs-1000cc.html