When choosing a saddle pad for your saddle it is important that the pad, under compression, not be more than a quarter of an inch thick. The pad may be an inch thick but if you take your thumb and forefinger and pinch down on it, you would like to see it compress down to a quarter of an inch or so.
Some of the worst are these bargain priced wool felt pads out there that can be up to an inch and a half thick and when you press down on them there is virtually no compression at all. The problem with these wool pads is that they are made of wool fibers that are packed firmly together to form the pad.
If your horse takes a tree with a seven inch spread in the bars, a pad like this will close in the gullet to five inches. The saddle may fit perfectly but your horse can come up sore because the pad closed in the tree so much it pinches your horse.
Picture what would happen if you had a pair of shoes that fit and then you put on an extra pair of socks. It would obviously change the fit to the point where the socks were causing the pressure, not the shoes.
So when looking for a pad, look at ones that have plenty of give to them.