If quadrupeds can carry more than bipeds of the same strength and size, then does a sleipnir with 6 legs carry even more than a horse with the same size and strength? What about giant centipedes? WHAT ABOUT GIANT MILLIPEDES?!
A sleipnir can carry 519 pounds as a light load, 1,038 pounds as a medium load, and 1,560 pounds as a heavy load.
This matches the expected carrying capacity of a large quadruped. So, It seems the number of legs doesn't really matter.
I think the answer is probably that carrying capacity has nothing to do with total number of legs in reality, and is actually biped vs non-biped issue (as well as size category).