A word in a muscle name that indicates that the muscle decreases the size of an opening is

"sphincter"

A sphincter is a circular muscle that maintains contraction unless, otherwise, required to relax as a physiological response. Examples of sphincters can be: the precapillary sphincters, the sphincter ani, the esophageal sphincter or the i ris sphincter. They are to control the entrance of liquids or solids, gases or even light.