Well, there are some items in the DMG that increase carrying capacity, though not in the way you're looking at.
Bag of Holding, Type I: 250 lbs, 2,500 gp
Bag of Holding, Type II: 500 lbs, 5,000 gp
Bag of Holding, Type III: 1,000 lbs, 7,400 gp
Bag of Holding, Type IV: 1,500 lbs, 10,000 gp...