Re: friction

Frictional force is a function of the applied force moving the object and 
the coefficient of friction.  Coefficient of friction is a function of surfaces
in contact and the area of that contact.  A greater area of contact gives a greater
coefficient of friction.  Another factor affecting the amount of friction is 
impulse. A great force applied in a short amount of time can facillitate a 
large frictional force while a small force would not generate as much 
kinetic friction.

Also, consider that those tires get pretty hot during a race.  If the tires are larger then
the heat can be absorbed in greater amount without overheat the tire. Friction is also a function
of temperature for some materials like rubber.  Hot tires will begin to melt and the coefficient of
friction will drop producing a skid or worse.