...yes, it’s normal to have one breast that is slightly larger than the other. The larger your breasts, the more obvious the asymmetry. For about 65 percent of us, the more voluminous one is the left.
Breast asymmetry is associated with rapid growth spurts prior to and during puberty. It’s normal for one breast to respond more to surging estrogen levels. Hormonal disruptions and other development problems accentuate the asymmetry. But why the left? No one knows for sure, but there are interesting theories...
I would think the nursing-related explanation is the most logical. (via the new shelton wet/dry)