Leather does stretch; in fact, leather belts in machinery will stretch about 6 percent over the life of the belt. You likely have not noticed it, but a belt that you have worn for years has likely gained a percent or two in length; but if it is too small, you’ll need to stretch it more actively. Any stitching or boarding will inhibit stretching; still, it is worth a try to lengthen the useful life of a good belt.
Another Hole
If you have the length, the simplest way to stretch the life of the belt (other than stretching the leather) is to punch another hole. Measure the spacing of the existing holes and mark where you wish to punch a new hole. Use a leather punch if you have one; otherwise, use a drill and drill bit for a perfect, round hole.
Saddle Soap
You may simply wet the belt and wear it. Water is not as poisonous to leather as you would believe, and a wet belt will stretch fairly readily. Use saddle soap, which will keep the belt from drying out and will condition it as well. Manufacturers usually advise against soaking the leather through, but you need to do so to stretch the belt. Wear it until the belt dries.
Alcohol
As an alternative, mix a solution of three parts warm water to one part isopropyl (rubbing) alcohol in a spray bottle. Spray the belt well, front and back, and put it on—a bit tight, if you can bear it. Wear it around the house until the belt has dried. Your body heat, and the evaporative property of alcohol, will help it to dry in a few hours. You should notice that the belt has stretched. Repeat as necessary.
Chemical Stretch
If the above fails, you will require a shoe stretching chemical. The most readily available are from Kiwi, such as its Universal Shoe Stretch, for smooth and nap leathers. These products break down the collagen in leather, which lends living flesh its firmness and leather its sturdiness. You use these as you would the water/alcohol solution, spraying the belt front and back and wearing the belt until it dries, repeating as needed. Despite the volatile organic compounds in these products, they will not harm you in contact with your skin.
Conditioner
A belt should not shrink with constant use, but a conditioner will ensure that it maintains its length. Use a lanolin-based conditioner, which will not stain your clothing as an oil-based conditioner will.