Start by noting that
(Load in pounds = (ST x 200)) is an absolute
maximum, achievable only under ideal circumstances: a healthy horse (or two), pulling a well-constructed cart/wagon, with properly balanced cargo, on a smooth, well-paved road.
According to most real world sources (such as
this one), a general rule of thumb is that a typical horse can pull a load (on wheels) of no more than 3 times its own weight (expressed as weight to load, 1:3), given the ideal circumstances above. If the road is bad or hilly, that drops to only 1:2, and only 1:1 on normal (non-road) terrain.
A full-grown draft horse can weigh between 1400 and 2000 pounds, which means a maximum load of 4200-6000 pounds using the real world rule of thumb. But ITL says a draft horse has an ST between 26 and 38, which would be 5200-7600 pounds, which is which is about 125% of what it ought to be, even for the ideal circumstances.
So I would fix it by saying the bottom end of the range is the base, and is
(Load in pounds = (ST x ~50)), with a 2x bonus for having a half-decent road surface, or 3x for excellent roads (which may be uncommon, depending on the setting).