From what I remember of high school physics ... the amount of friction is determined by the 'coefficient of friction' and the 'normal force' (downward force), and it has nothing to do with the contact surface area ...

So ... that might lead me to wonder if deflating a tire ...

(1) changes the coefficient of friction - which seems kind of odd

OR

(2) doesn't really increase friction, but it does makes the tire less likely to sink into a soft surface, which really does help.

OR

(3) just works, and I clearly remember too much from high school physics for my own good.

Sigh. My brain hurts. I think I'd better stop posting here for the rest of the year ;-)