[BBray_T1296]

Larger wheels/more contact patch supplies more grip due to the "cleating" effect where the "pinion" of the wheel has physical exertion on the "rack" that is the carpet. Like gears, there is more than static friction meshing the treaded wheel to the fiberous carpet.

If you were only just calculating straight static friction, wheel diameter and even wheel number has zero effect, because while you are increasing contact surface area, you decrease weight per unit surface area in a 1:1 fashion. All that matters in that calculation is coef of friction and weight of robot (f=mu*N)