Thinking about a wider rear tire

[Pterodactyl_imp]

^^^^. Correct. But, that formula calculates the coefficient of friction between two surfaces. Not the total of the friction that exists between two surfaces that would need to be overcome by a force before break away occurs. That is traction.

Simply put, for any given tyre compound at any given temperature on any given surface, the greater the contact patch the greater the traction. Therefore tyre size and shape, all other things being equal, determine the traction.

The temperature of a tyre also helps determine the amount of traction provided. As the tyre gets warmer the compound becomes more compliant. This increase the contact patch. Thus tyre earners. This increased traction continues until the compound exceeds its design temperature, causing the tyre to leach oil or the tread separates, or both.
Cheers