Don't forget to add the rider's equipment, too. I had someone do that: "I weigh 200, my horse can carry 233, it'll be fine". This does bring up some entertaining possibilities, like where the Half-Orc can carry more weight than his horse can, especially when you count magic items.
That's when you invent something like the Saddle of Hauling (+4 enhancement bonus to STR only for the purposes of carrying capacity, only works on mounts, counts as a military saddle)
Frankly, though, the only people I ever see pay any attention to their mounts are the Paladins. It's too bad, really.