Will my boobs get a lot smaller if I lose weight?

(This comes from a gifted 19-year-old who would like to hold on to what she's got, as it were.)

I'm afraid you can't pick and choose where you are going to lose weight. The more weight you have to lose, the greater decrease you may see in your bustline.

Honestly, from the photo you sent, you could lose a good deal "upstairs" and still have plenty.