In one of her live videos she says that they change in size throughout the month. I looked at her most recent Xmas post on reddit and they look as large as they always do. It would be pointless to get implants for such a minor size upgrade when they're already so large.
Fluctuations in boob size? Not really, hormonal changes. Can affect some people more than others. Esp if she's on contraceptives as well, a lot of them work by regulating hormones.