Despite what we see in lad mags or view women who have boob jobs, 90% of women's boobs are not symmetrical, either by size, volume or position. With the left side bigger in the majority of women (due to the heart position) by up to 10-15%. With 25% of women having breasts that are a cup size different.
Though when it comes to buying an off the peg bra, the cups are the same size solutions to this problem is:
1. You can surgery to correct this if it's truly noticeable and you're not happy or feel self conscious.
2. Become a lingerie designer and invent a bra where as well as altering the straps and underband as we have today, you can alter the cup size. Lorna Drew did this for maternity bras.
3. Embrace it, go braless, and celebrate that you're actually just like 90% of women out there. And take note that no one really cares or notices.