If the saddle sits behind the 18th thoracic vertebrae, then no amount of padding is going to help you. In simple layman's terms, once the saddle crosses that point, it's no longer sitting on the rib cage. Even shock-absorbing pads like Thinline will do nothing for that problem.
The delightful converse to this is that even if the saddle LOOKS big to you, if it fits correctly and it's not beyond that magic 18th thoracic vertebrae (or it upsweeps in the rear so that the saddle is not making physical contact behind the 18th thoraci vertebrae), then it probably won't cause soreness. My little 15.3 hander ran around all summer in an 18" saddle (trainer was 6'2" and skinny), and while it looked disproportionate and therefore kind of stupid, the saddle sat entirely on the rib cage and therefore we had no soreness problems.
My Quarterhorse is 1 inch short to the 18th and I needed an 18" seat. I ordered a saddle with upswept panels ( based on his tracings) so that the bearing surface ended in time. Black Country is one of many that can do this.
Thanks! I will double check on the vertebra. The thing is, originally I thought it fit well, then I started noticing dry spots, so then I pushed the saddle back more, thinking it was too far forward. (Big shoulder, big withers, curvey TB back.) That fixed the dry spots, now I'm afraid that the saddle is too far back!
Argh. I have saddle fitters I can contact in CO, but I am in NM until fall. It was tough to find a saddle that fit us both, I am cringing to think of more saddle shopping.