Horses are big animals that can range from under 800 pounds to over 2,000. How do you weigh them? I mean, where do you find a scale that big?
The answer to the latter is rural weighbridges, but not many horse people have access to those. In fact, horses are generally "weighed" as a calculated estimate based on the heart girth (immediately behind the elbow and withers) and sometimes the length. Also, very experienced horse people can often eyeball a horse's weight with surprising accuracy. (And no, height is not in there).