It doesn’t. The weight of a horse in horse racing remains fixed by a number of legal factors: The size of the horse.
The height of the horse.
The weight.
The number of wheels in the horse.
The weight of the rider. If it was changed because you weighed it too low, the result would not be “sportable” any more; it would simply be “overweight” — a racing horse is no different than a car that weighs more than 80,000 pounds at full speed on a dry surface. To make a horse competitive, there had to be something other than gravity on its side — something called “race-day weight.” Race-day weight can add more than ten pounds to a horse’s weight, so it would normally not be weighed, but some racetracks, like Michigan Derby, let the warden weigh the horse at check-in. Some fields allow racers to weigh the horse at the track, and when a horse is weighed before a race, it must be weighed at the track in addition to any weight it has already gained before race day. The weight in one of these places is called “race day weight,” and it is the average weight of the horse in the race. The warden can only weigh the horse on race day if it is weighed by the race director or any person or agency authorized by the race director to weigh an animal. If the warden weighs the horse at a race track where horse weigh-in is conducted on his own time, he has no authority to weigh the horse before a race. On the other hand, if the warden weighs the horse at the track, his authority to weigh the horse starts when he arrives at the race and ends when he leaves. A warden’s weight may be “banned” by the race director (the race director’s right to weigh the horse at the track) or he may not exercise his authority to weigh the horse. An exception to this rule is the racing warden at the Kentucky Derby, who must weigh the horses before and after the race. The warden can weigh the horses at their weigh-in if he is required by rule to do so, but he cannot weigh the horses during the event. He can, however, weigh the horses before each race, if the horses weigh at home at some point at previous races in the season. If the warden weighs the horses before each race, it is still common for a race director to weigh the horses
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