Some people are by nature small chested. You are petite all the way around and it is unusual to be very petite and have a large bust. You get your period on a regular basis?
Have your endocrine system check. It can see if it's a hormone problem or Turner's syndrome.
Some people are just naturally skinny. I have a friend who is the same way. Its nothing to be worried about. Insecurities are hard to overcome but just remember every body is beautiful!