Why is one breast larger?

See below: Most women have assymetric breast size.

Breast size: It would be more unusual to have breasts of identical size and appearance.

Nature: The human body like much of the rest of nature is often asymmetric.

Common: A small difference in breast size & contour is common.It usually becomes evident during puberty when one side progresses sooner than the other. In less common cases it can be sizable. If the difference is enough to interfere with your life I would seek an evaluation by a surgeon who specializes in breast augmentation/reconstruction.