I had a saddle with a 20" seat (very comfy), and an 8" gullet. Saddle popped up in the rear nearly three inches when we cinched it down. Dancer didn't care, but I did. Daughter tried riding in the saddle (seat was way to big, but it was Dancer we were worrying about) but when daughter sat in the saddle, it must have pulled on the cinch, because Dancer really started pinning her ears, and the cinch did look uncomfortable.
I sold that saddle. I have a 17" saddle with a 6" gullet that fits her pretty well. However, I haven't ridden her in it, so I don't know if it will work for her with me in it. I rode poor DJ in it - he didn't seem to have any issues, and my older daughter used it on him when she and younger daughter went riding together. It looked like it fit him pretty good. DJ had moderately high withers.
The saddle is built a little oddly, IMHO, because it may only have a 6" gullet, but it really flares out from that point, and the pommel is cut up way high, giving even more room for Dancer's withers. I was going to try the saddle on her today with no pad to run my hands under it and see if it will really work for her, but it was pouring rain and she was soaking wet. I'm not so picky that I won't take my saddle out in the rain, but I had already been out there long enough that I was pretty chilled. (Farrier was out today)
Plain Old Dee, horses Dancer and Rain I believe in dragons, unicorns, good men and other mythical creatures!