Specification
2006 Dodge Ram 1500 5.7L Laramie
140.5 WB
wheel sizes permitted P265/70R17, or P275/60R20
GVWR 6700 lbs
GCWR 13000 (20"), 14000(17"),
Axle ratio 3.92
4x4

Reference documentation - http://www.rambodybuilder.com/2007/docs/dr/mlup.pdf

17" wheel / Payload 1230 / GCWR 14000 / Max Trailer 8400
20" wheel / Payload 1160 / GCWR 13000 / Max Trailer 7300


Question
My vehicle is fitted with 20"/9" wheels and it shows above the maximum GCWR is 13000lbs and the Maximum Trailer i can pull safely is 7300lbs however, when you look at the same vehicle with 17"/8" wheels the GCWR increases to 14000lbs and the trailer weight increases to 8400lbs.
What makes a 17" wheel more capable to tow a trailer at a higher weight than a 20" wheel?