I know that western [riding] saddles for horses are different (have different trees??) from such saddles intended for mules. I know that custom-built saddles can be extremely expensive. To simplify the thread, let's limit to western saddles for horses. How do custom built western riding saddles adjust to horses' backs of significantly different sized horses? I suspect putting a heavier blanket padding between saddle and back is not a satisfactory method of adjusting since a saddle tree is probably built for a relatively small variation in horses' backs sizing.