Saddle fit can be that huge a difference, I've had horses with bucking or other issues and sometimes just re adjusting or changing tack the problem disappears. Some of the new full qh trees are super wide and only fit the roundest heavy built QH types, average built well fleshed horses are usually more regular QH tree and your narrower breeds, STB, Arab, TB and light build crosses should be semi qh tree, but even then each saddle manufacturer may have slightly different specs for what they call semi, full or regular and even worse gullet width doesnt always indicate tree size, sortof like having a size 8 boot, but is it wide, narrow or extra wide, flat insole, raised insole, narrow toe, square toe, round toe. Still size 8 boot but will all fit different depending on what type it is.