If not washed properly, denim does have a chance of shrinking. Heat and water can cause the shape of the jeans to change.
Jeans have a tendency to fade or shrink over time if the proper precautions are not taken. Adding salt to laundry can help prevent fading and shrinking, especially in black denim. Jeans will last longer if washed in cold water. If drying them in a machine, use the lowest heat setting. It is possible to stretch the jean if they have shrunk. Place a board in the waist band while the jeans are damp to make them return to the original size.