First: some experts say that you should buy a saddle slightly larger than your horse's back, then the saddle should be shimmed and padded to create a flexible custom fit for your horse's back, like buying your shoes a 1/2 size too big and putting custom inserts in the bottom to fit your individual foot. Proponents of this fitting method say that your horse will be more comfortable, and have a less restricted range of motion because they have the flexibility of the padding between them and the rigid structures of the saddle tree.
On the flip side, other experts say that a well fitted saddle shouldn't require a pad at all, and that it will not interfere with the horse's motion IF it is correctly fitted and placed. The argument also runs that a badly fitted saddle is a badly fitted saddle, no matter what kind of padding is under it, and will never be as comfortable for your horse as a properly fitted saddle. By this theory it is nearly impossible to get a saddle without having it custom made for your horse, using multiple measurements, NOT just wither tracings.
I can see the logic of both ideas. I haven't figured out one or the other of these theories applies more to western or English saddles, or vise versa. I would love to hear what others think.