Besides the fact that the original post asked about mounting slicks that were not necessarily wider, here is an explanation as to why wider tires offer more traction (even though the physics that TheContourGuy mentioned is correct...)
Let's look at the history of drag racing.
The first "slicks" were truck tires that had the grooved tread shaved off. They used these for a number of reasons. Two of the reasons were: one, the taller tires added gearing to final drive ratio and; two, they were wider and spread the load over a large area to gain traction.
"Huh?" you say, "How did number two work?" .
Well, let's look at the road surface. It's not perfectly smooth like the laboratory table, as a mater of fact, it's rather rough and porous. A thin tire will spin and the road surface will fill with rubber from the melting tire, when the pores are full of hot melted rubber, the surface tension (i.e., friction) is reduced and grip is lost. However, if you spread that over a larger area (i.e., reduce the surface tension per square inch by spreading the pressure), it takes longer to fill and the surface tension is maintained longer. The longer you can maintain the surface tension, the faster you'll launch.
Theories are for perfect worlds... not practicality.
