Actually this is something we used to tune with a lot about 10 years ago. You can narrow them and widen them.
You can make them narrower by cutting the inner bead off right against the inner band and mounting the tire inside of that.
You can make them wider by cutting off the outer band on the outside bead and mounting the tire on the very edge of the wheel.
Obviously there is room to experiment with this and moving the tire around in many combinations.
Keep in mind ALL the things you change though. Not only the width of the tire. The wheelbase changes, as well as the ride height. It can effect offset and even roll center.