Ok, there’s been way too many people freaking out about their waist size. To gain any appreciable amount of muscle, you must gain fat. Accept this.
However, if you desperately wish to slow the expansion of your waistline, you can strengthen your transverse abdominus, the muscle that controls how much you can suck that “gut” in, by doing stomach vacuums.
To perform them, you breathe all of the air out of your lungs and suck your gut in as hard as you can. Hold it until you can’t any more and repeat. Keep trying to progress by holding that “sucked-in” state longer and longer.
After you have done vacuums for some weeks, you will be able to hold your gut in much easier and hence a smaller waist.
DISCLAIMER
This will not reduce the amount of fat on your waist nor will it reduce the amount of fat you gain. It will merely make it easier to hold your stomach in just as deadlifts make it easier to maintain good posture.