A properly fitting saddle is crucial for both your horse and yourself. If you don't get it right, you may end up with back pain or slipped discs. The fit of the saddle can't be compromised - the comfort of both of you is essential. It's also important to consider the size of the rider's girth. It's also important to make sure that the girth/cinch is tight and does not slip. A loose cinch can lead to a fall and injury.
Source: https://saddlemania.com/