General rule of thumb, the larger the rider, the longer the saddle to fit the rider's behind. If the saddle is a very good fit then the horse can carry a heavy rider. But a very good fit isn't the norm. The padding of an engish, nor the bars of the western must not go past the last rib or the horse will develop back issues or may buck, etc. There is a lot of movement behind the last rib which can push the saddle forward or to the side. If the saddle goes forward it can damage the ligaments in the shoulder blades.