Gaining

Probably they just seem smaller with the rest of her expanding. As the band size increases in her bra, the cup volume will also increase. So, her cup size could actually go down if her breasts stay the same.

With some women, a lot of the weight gain goes to the breasts. With others, when extended pigging out occurs, another part or parts of the anatomy are the beneficiaries.
I like it when everywhere on a woman expands with the added pounds. An overload of erotic flesh all over is where it's really at, though the other body types, where the additional fat is more concentrated in certain areas over others, have their definite sexual attraction too.