Why does one breast grow more then the other??
ANDY'S ANSWER: Lots of parts of your body grow at different rates, and it's not uncommon for the same kind of parts of your body to grow at different rates. This is called "differential development".
Even after everything is finished growing, you will not end up with everything perfectly the same on either side either, although it's unlikely that any one else but you would notice the differences and even you might not see the differences.
As a matter of fact, there is not any body that is perfectly symmetrical on both sides. If you want to test that, just take a picture of someone's face, split it into two halves, duplicate and flip each side of the face and put them back together again. You will find that you have two totally different pictures of that person.