If you're overweight and have pretty huge breasts then you'll see a noticeable difference in the size of your breasts once you start losing a lot of weight.
If your breasts are not all that big to begin with then you might not see much difference in breast size when you lose weight.
I wasn't really overweight but I did drop 40 lbs and did notice a little bit of reduction in the size of my breasts but I didn't notice too much difference.
The bigger your breasts to begin with the more weight you're gonna lose the more you'll likely notice about the reduction in the size of your breasts.
Breasts have some fatty tissue in them so along with weight loss your breasts will usually become a bit smaller.