6 months post op and my left breast if bigger than the other one. What can be done to correct this?

The difference is present, but is mild and was prior to surgery. You could consider fat grafting to the smaller breast. If you have saline implants, more could be added to the smaller side. If you go up to the next size in gel implants, it is possible that the smaller side would then be larger. You may not be able to get them exactly alike since they are two different breasts. Please discuss your options with your surgeon.

You do have preexisting asymmetry.  Your breasts are different shapes and one nipple is lower than the other.  This would be a tough case to achieve symmetry in any surgeons hands.  Unfortunately when you get implants is often makes the asymmetry more noticeable.   I suppose you could have revision to address this but I’m not even sure what to do.  If you raise one nipple to the height of the other it will entail scars that you would really have to consider carefully.

Dear Loving9670,
Women’s breasts have some degree of asymmetry. Some may be more noticeable than others. If you have breast asymmetry before breast augmentation surgery, there is a great chance that this will exhibit right after the surgery.If the asymmetry becomes more noticeable after breast augmentation surgery, a revision may be needed to correct the asymmetry. In most cases, the revision involves correction in both breasts. For example, large breasts can be reduced to a more manageable size, but the amount of reduction might differ between each breast if one happens to be naturally larger than the other. The same can be said if a person wants to augment their small breasts to a larger size: one might be augmented more than the other.You should discuss your concerns with your plastic surgeon. Make sure your surgeon understands your goals. 
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery