Why is there so little skin in between your navel piercing?

That will depend. If it was recently pierced, then that may be because the swelling has gone down. But if you've noticed the skin getting thinner and thinner, it may be that your piercing is migration, which means that as it heals, the body is pushing it outwards. In that case you may need to have it checked to prevent it going on until the piercing falls out.