The depth of a belly button fluctuates with the size and shape of an individual. A person of large girth often has a deeper belly button than someone who is smaller. This also means that the depth of a person's belly button sometimes changes over time.
A belly button is a scar left behind when the umbilical cord is severed after birth. Belly buttons, much like fingerprints, are unique. Variations in size, shape and depth occur over the course of an individual's life, as well. As human tissue stretches or contracts, so does a belly button. Giving birth often also changes the shape and depth of a woman's belly button.