There is no such thing as a saddle that will fit any horse. Horses, like humans, come in different shapes and sizes. A horse that has a broad, flat back and little wither will require a vastly different saddle than the narrow, slab-sided horse with a high wither, especially when you consider the fact that the entire panel must conform to the horse’s back. “Padding it to fit” simply isn’t good enough (unless you’re expecting the horse to change, in which case a slightly wider tree with carefully shimmed pads is acceptable). It would be like asking a very petite person to wear a huge pair of pants, saying that a belt should suffice in keeping them up. While this may be true, the pants will be bulky, the legs too long, and chances are the person wearing them won’t be terribly comfortable (and, of course, they’ll look funny in the process).
Since we are asking our horses to not only bear our weight, but also perform maneuvers while working in a collected frame, (jumping, lateral movements, etc.), it is up to us to ensure that they are comfortable so that they can perform at their best. If your horse is not comfortable, he cannot carry himself correctly, and if forced into a frame while he’s uncomfortable, he cannot help but put added stress on his legs, let alone try to function with a sore back. The fact that most horses perform under uncomfortable, ill-fitting saddles with stoicism only solidifies the need to pay attention to how the saddle fits the horse before the vet bills begin to accrue. Oftentimes those subtle changes in behavior or movement are dismissed as the horse’s bad attitude. Ask any equine behaviorist and they’ll tell you that horses are generally agreeable beings with a deep desire to please the humans in their lives.
Using one saddle on every horse, “padding it up” as necessary heightens the chance that you’ll end up with a lameness issue and the potential for an expensive vet bill. When the saddle doesn’t fit right, there are pressure points. When there are pressure points, there is a reduction in blood flow to the muscle, and over time you will see deep tissue damage and ultimately muscle atrophy (especially evident around the withers from an improperly fitted tree). Pressure points not only affect the horse’s back, but also the rest of his body, especially the legs. Think about it: if he’s uncomfortable because of a pressure point digging into his back, he’s going to instinctively brace himself against the discomfort. That bracing ultimately adds extra strain to his legs and joints, and you can wind up with arthritic changes, suspensory and ligament injuries, and muscle tears.
Making sure that each horse has a saddle that fits properly is a small price to pay to ensure that our equine athletes are comfortable, safe, and sound. You wouldn’t try to make one pair of pants fit everyone in your family; why do the same thing with saddles? 
