Correct tongue posture has more to do with your midface and forward projection of your jaw than the size of the chin itself. While mouth breathing / incorrect tongue posture can destroy your midface and jaw projection, I don’t see how it can affect the size of one’s chin. Chewing hard foods will only cause your masseters to grow, which will make your jaw wider, but doesn’t really do much for your chin. Is it determined solely by prenatal/pubertal testosterone and are chin implants to only way to fix it?