Mike - GM has 3 groupings, light, medium, and heavy duty [really big], but there's only 2 variations in the sheet metal - "big truck" here means 1.5T and above, the light trucks [up to 1T] have 15-19.5" wheels and all the same size fenders, 1.5T and up use 20-22.5" wheels and have the larger fenders
Bill