I worked with a Paint like that before, saddle fit her just fine, it was just the fact that she was just completely round, like a barrel with legs and a neck sticking out. The saddle slid around EVERYWHERE. We put on a breast plate/collar and a crupper. I also used a very thin pad, if anything at all, besides the anti slip pad. Usually I actually only rode her with that pad. We also did a lot of trail work (trotting up hills, etc), and that seemed to help her build some better muscle that "held" her saddle better. Hope it helps!