Most people only check in one location on the horse's body when determining the tightness of a girth or cinch - just above the horse's elbow. But horse's can have different conformation in this area. This can affect the perception of the girth or cinch's tightness, and result in a false reading as to its true tightness.
As seen in this clip, I advise checking the tightness of your girth in two locations: just above the horse's elbow, and where the girth contacts the sternum. Doing so will provide you with more accurate information as to the tightness of your girth or cinch, allowing you to make decisions that will enhance your horse's comfort and performance.