If you use lip fillers to the extreme, or if you choose to use untrained injectors, your lips can become permanently stretched or damaged. This means that your lips may or may not return to their normal proportions once you stop using lip fillers. If you have been going to a trained and specialized lip injector and have been sticking to the recommendations they have provided, your lips should be fine to go back to their normal proportions, which would be your lip size before you began lip injections.