For those of you riding ponies or shorter backed horses, saddle length can be an issue. And it's a tough one. You want to distribute your weight over the biggest (but still weight bearing) area possible, but you don't want your weight to interfere with shoulder action or rub the horse's hip when turning. My mare Ava is 14.1H and while technically some might call her a pony, she still feels horse sized, or at least not limiting, for saddle length. But she's the shortest back of my test horses for comparison.
She was ridden in a 17" Roma for 2 years of endurance competition, including two 75 mile rides. The 17" saddle has a 21.5" panel.
The 18" saddle has a 23" panel. The last couple inches under the cantle aren't truly weight bearing so unless your horse rises sharply to the loins, there's a good chance the saddle may still work for you.
It's more important that the rider be in the right seat size, since the cantle isn't meant to have seat bone pressure, we don't suggest going a smaller seat. Although there are times when I think someone would be more comfortable in the bigger seat, the width of their seat bones dictated the smaller seat worked better.
This is one of those things that you can't tel until you try, another reason I very much recommend a demo. No reason to buy a saddle and have it not work for something minor!