Diet and consult MD. This comes down to an analysis of the differences: it may be simply making the larger breast smaller or the smaller breast larger. However it more likely involves a combination of both as well as analyzing the need for a lift or repositioning of the breast creases or nipples or parenchyma. Perfect symmetry is not a realistic goal and will set you up for disappointment. Goal: less asymmetry:).
Breast assymetry . Asymmetric breasts can be corrected through a combination of lifting, reducing or augmenting. Your weight does need to be stable though in order to have a long-lasting result. Weight fluctuations are one of the top reasons for breasts losing their fullness and sagging.
Plastic Surgeon. You should have a consultation with a board certified plastic surgeon to determine the best procedure for you.