The fender and running board width is indicative of the weight rating of the front axle.
Narrow fenders are on trucks with "lighter" front axle weight ratings and, conversely, trucks with wider fenders and runing boards have "heavier" front axle weight ratings.
I don't know what the cut-off is (example: <7000lb axles = narrow fenders/running boards and >7000lb axles = wider fenders/running boards) but that's the reason for the narrow versus wider fenders...
