First of all it is not guaranteed. Only some women will experience this. Normally it will be a cup size or less, so you are not going to go to DD from an A cup, if it increase at all.
It will normally reduce again after stopping. Speak to your doctor about it. There are various low dose pills and other methods available that will reduce all side effects, including this one.
Normally the slight increase in breast size is experienced very positively by both the woman and her partner. It is very very very seldomly an issue.