Can you tell me if a girl loses a lot of weight, will her breasts shrink a whole lot?

Possibly: Each individual's body stores fat in unique ways. That said, women tend to store extra fat in their hips/legs and breast (hence the "pear" shape) versus men who tend to store more fat in their abdomen ("apple" shaped). It is possible for breast size to decrease with significant weight loss, but the degree of shrinking is difficult to predict.