Does your lip piercing size shrink if you were to put in a smaller stud in?

Yes. When smaller jewelry is put into a piercing that was pierced at a larger gauge, the piercing will shrink slightly to accommodate it.

However, you must be careful when shrinking piercings. If you put in a smaller stud or piece of jewelry then the hole size, it can sink inside of it. Lips are notorious for having small studs sink inside. Ask a piercer for advice when downsizing piercings to be sure that sinking doesnt happen. Sometimes just putting a bigger ball on the end of the stud will keep a smaller post in a piercing, so it doesnt sink.