Most women have slight differences in sizes or uneven breasts, in the same way that most of us have one foot or hand a fraction larger than the other.
All humans (male and female) have a tiny nodule of breast tissue present behind the nipple from birth. At puberty, the oestrogens in women stimulate this tissue to grow into a breast. The degree of stimulation, the size of the original nodule, and (most importantly) hereditary tendencies will determine your breast size. In some women the breast tissue on one side does not react as much to the stimulating hormones as the other side.
Some women are born with no nodule of breast tissue behind one nipple and therefore there is nothing there for the hormones to stimulate at puberty.
There is no magical medication, cream or diet that will correct uneven breasts, but plastic surgery may improve a woman’s self image and appearance dramatically.