How do you make your girlfriend breast big?
The two most common ways are to either pay for surgical implants (which would require her permission and acceptance) or to get her pregnancy (which should require a discussion with her beforehand, since having a child is a life-long commitment). Pregnancy and lactation will cause development of the breast tissue, making them larger for a period of time. However, after a woman is done breast-feeding her child, her breasts may return to their pre-pregnancy size.
I think a very important question to ask is, why do you want to make your girlfriend's breasts bigger?