When a rider is too heavy for their horse, the horse will suffer. They will show increased stress levels and will develop pain in the back and even transient lameness. Over time the damage will become permanent, and the horse will no longer be sound enough to ride.
A lightweight tall rider is unlikely to cause physical damage to the horse. However, the rider’s riding will suffer due to compensatory riding habits. Tall riders will develop habitual leg lifting when applying leg aids or ride with very short stirrups.
A rider with short legs on a broad horse will struggle to get their leg around the horse’s barrel. An enormous strain is placed on the hip flexor and adductor muscles. The rider will arch their back to relieve stress on the tight muscles.
The topic of rider weight is sensitive. There is no definitive rule to enable an objective judgment of a horse’s weight-carrying capacity. Multiple variables influence a horse’s ability. No single formula accounts for all of these variables.