My gelding has great withers. Makes it hard to find a saddle that fits, but when one does...it stays. My daughter once finished a ride, and it was slack - as in daylight on the bottom. She & I had some words after that, because that was dangerous, but I LOVE horses with withers!
Beyond that, some horses will bloat, and some saddles will settle. I generally walk the horse some to reduce the effect of bloating, but the saddle will still settle. I find that if I tighten it tighter than most recommend, it will be slightly loose after 10 minutes of riding. I usually apologize as I tighten it: "Sorry, boy, but we both know it will get looser as soon as my fat rear settles down on it - so the sooner I'm up, the sooner it is loose!"
To date, I have no evidence he understands sentences. Oh well.
Also - what works for me is one hand on the withers, and one hand on the far side of the cantle. Make certain you are close to his side. The farther from his side, the more torque when you go up. Hop, hop, hop, then up and lean a bit over to the other side, so my weight is centered on his spine ASAP. THEN swing around and settle.
If a horse isn't inclined to wait patiently, then I figure he deserves an extra tight cinch and a quick but less comfortable mount. Even my spooky mare has figured out that if she stands still, I'll get my weight centered quickly and it is easier for her.
BTW - I also LOVE short horses. My mare is 15.3, and that is about 0.3 taller than I would buy again. I'm 53, and gravity is stronger than it was in the 60s & 70s. That is what has killed the space program - increasing gravity. The mountains are also getting taller, rocks larger and harder, and some of the trails of my youth have doubled in length. I wish a scientist would study it...