anisomastia anisomastia anisomastia is a medical condition in which there is a severe asymmetry or unequalness in the size of the breasts, generally related to a difference in volume. in other words, when one of the breasts is much larger than the other. in contrast to anisomastia, a slight asymmetry of the breasts is common. anisomastia may be corrected by surgical breast augmentation or reduction.