You know Brian, that's a good question...
If you read about the middle of my AP build up thread, George talks about (suggests) narrower tires for "more" traction...
It comes down to contact area and weight on axle/tire
You only have so much weight on front and rear axels...a narrow tire will have more Lbs/sq.inch (pressure) and technically should "dig in" and have more traction..as well as have less rolling resistance "pushing" thru the loose stuff...(per low horse power)
Although, if you have enough "weight" on the axel...(rear?) a wider tire at the same lb/sq.inch ratio should have more "gripping" area...
But then think of these scenerio...
Two big wide tires on the rear...solid axel...lots of weight..."IF" you have "too much" traction on rear..it'll be harder to steer...overpowering front...
Or look at trucks in the winter..which have better traction...little "skinny tires digging in and moving snow...or the big fat "mudder tires" that float and slip and slide...??
On a high traction surface like say asphalt or "sticky" wet clay...wider tires offer more "contact" and traction..but we're often racing on loose dirt and grass...
Gonna be interesting... Hopefully we'll get more input..