Saddle Fitting is a science and an art.
There are many considerations when you start fitting a saddle to both the horse and the rider. Horses and riders change enough over their lifetimes that rarely one saddle will fit both of them for long periods of time without adjustment.
Saddlers have been using science and art to find better ways to help saddles to fit both better and longer.
English saddle makers have made serious accommodations for shoulder movement, by keeping the points of the tree as high as possible. While this takes care of one problem it only move the pressure of those points higher up on the horse.
In addition to these high points English saddle makers flock their saddles to the shape of the horse's back that they are building the saddle for. The flocking still raises the rider up off of the horse many times over an inch.
Each time your horse's back changes which can be quite significant when a horse is in training, can cause your saddle not fit, slowing your progress if it is causing pain. Also adding the expense of re-flocking your English saddle regularly ensuring your progress is not delayed.
Western saddle makers have also made many changes to their saddles to help with these shortcomings of rigid tree saddles. Incorporating twist and flare in to the bars of the saddle have helped with the pressures exerted by a rigid tree.
Western saddles have fleece so corrective pads and shims are used to obtain a better saddle fit if it already fits adequately. Shimming can't help a saddle that is too small for a horse fit better.
Imagine that you have a pair of boots that are beautiful and are just a little bit small for you. Would you add any extra socks to help them fit you better?
A saddle that is too narrow can not be made wider or to fit better with any kind of pad.