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Usually breast size will differ but not very significant difference in size. If there is much difference you will need to consult Surgeon to get evaluated for any lumps in breast. You may drink more milk, eat pulses, eggs, chicken, lean meat, eat methi or fenugreek in diet to improve breast size. Also wearing underwire brassiere will help increase the visible size. Join a Gym and learn pectoral chest muscle exercises
to improve breast size and definition. If you want definitive treatment you can consult Breast Cosmetologist for breast implants.