Overall, no one really knows what causes breasts to develop unevenly. It is usually down to hormonal changes or trauma to the chest but it could also be an underlying medical condition such as Poland syndrome or juvenile hypertrophy.
Juvenile hypertrophy is very rare but it does cause one breast to grow significantly larger than the other. Poland syndrome is again another rare condition that occurs at birth and one side of the chest wall is underdeveloped compared to the other.
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