A seat too small is uncomfortable, inefficient, and actively works against effective communication with the horse. The rider cannot sit correctly in a seat that does not accommodate their body.
A seat too small forces the rider to perch on the back of the seat or sit with seat bones against the cantle. This drives weight backward, tips the pelvis, and makes it impossible to sit in the balanced position that effective western riding requires.
Sit in a straight-backed chair with feet flat on the floor. Measure from the back of your knee to the back of your hip. Most adults measure 15–17 inches. However, body proportions vary — a rider with a long torso and short legs may need a larger seat than the measurement suggests. The only reliable confirmation is sitting in the saddle: you should be able to place a flat hand between your thigh and the front of the seat jockey with approximately one to two inches of clearance.
A rider transitioning from a cutting saddle to a reining saddle will often feel the reining saddle is too small, because the flat reining seat does not have the cutting seat's depth. Evaluate seat size in the specific saddle type you are buying, not based on size in a different discipline's saddle.
David Solum has been evaluating saddle fit problems for 40+ years. Call, text, or email him directly — he can advise on whether it's a fit issue, a tree problem, or a saddle you should replace.
See also: Free Saddle Tools · How to Fit a Western Saddle · Parts of a Western Saddle · How to Buy a Certified Used Saddle