Yeah... breasts are mostly made of fat.
And I've posted this many times before, but the number of fat cells in the body does not change. When you lose weight, all that happens is that the existing fat cells in your body all shrink, and when you gain weight they all inflate.
So basically, yeah, when you lose weight, your breasts will shrink in proportion to the rest of your body. Just like they'll grow in proportion when you gain weight.
The only way to gain breast fat and hip fat when compared to other body fat is time... hormones influence where new fat cells are created. So as the old fat cells die (about 10% die each year) and need to be replicated somewhere else, slowly they'll relocate to gender-appropriate areas. The speed of this depends on how readily your body's cells are replicating, genetics, and other things. (And this is why it takes cis-women like 8 years to completely shed the teenager-y look and develop full womanly curves. Because it's not just "yay, estrogen, I'm going to store all my fat in my boobs and hips now." It takes years for the childish fat-storage areas to relocate to adult female areas.)