For a western saddle: All western saddles extend over the loin
The design of an English saddle is very different. I would be curious to know if any studies have ever been done to show that it is harmful for even an English saddle to extend somewhat further back - although the overall length of the saddle ought to make that less common.
It isn't too hard to figure out where the front of an English saddle goes on my horses, both of whom have defined withers and shoulders. From that point on, it is what it is. About the only adjustment I can think of would be to either restrict the horse's rider to one who fits a smaller saddle, or to use a forward seat and keep the rider's weight more to the front.
Trooper has an uncommonly short back - 3/4 Arabian, about an 800 lb horse, 14.2 hands. This picture came from just after he arrived and shows both what a poor fitting western saddle did to his withers, and a ****ed ass with spurs did to his sides
...but Trooper acts quite comfortable when ridden in my 18" seat English saddle.
He also does well with a 16" western saddle (Circle Y Mojave). Food for thought...
If you are doing some jumping, I assume you use a forward seat. In that case, very little pressure ought to be on the rear end of the saddle.