How much smaller will my breasts get after losing weight?!

It depends on a lot. You are correct to expect that your breasts will get smaller if you lose a notable amount of weight.

How much smaller they get is directly linked to not only how much weight you lose, but also to your genetic predisposition on where your body wants to burn the excess fat. If two people of the same height and weight and cup size lose the same amount of weight, one of them may notice a big change in breasts, while the other may not. Genetics matter.

However, your cup size is determined by the difference between your bra-band size and your breasts themselves. Since you will also presumably lose some weight from around your rib cage, your cup size may not change as much as you think.

Here's my experience, but it may be different from what you will experience (remember genes): I've lost about 26 pounds. From around my breasts at the fullest part, I've lost 3 inches. From around my chest where the bra-band goes, I've lost 3.5 inches. The end result is that I buy a different bra-band size (the number) but still wear the same cup size. As for how it looks/feels... I can tell my breasts are a little bit smaller, but all my husband notices is that my entire torso (and body, of course) looks more trim and shapely. In comparison to the rest of me, my chest looks proportionately about the same now that there's less of me all over.