So are testes. Though we’re not entirely sure why the left breast is often bigger. And no, that is not a self-portrait.
While it is typical for women to have one breast that is larger than the other, if they are REALLY REALLY different in size, she should check in with her healthcare provider for an "is this okay?" talk. Also, if a woman notices that her breasts change in size after she is an adult and shouldn’t really be expecting any more changes in size, she should also check in with her healthcare provider for that or any other breast-related concerns.
You can learn more about breast health here.