If you lose weight, some of this weight is going to be fat and part of your breasts is made up of fats, so by reducing body fat you will reduce your breasts.
women who use steroids are typically obsessed over this shit so they drop their body fat super low and that is what shrinks breast size.
and frankly if your body fat is over 20% as a female (just like how males should be under 12%) then using steroids is pointless in the first place. and those body fat numbers are being super generous.
Breasts are primarily fat so loosing body fat will shrink the size of your breasts. Female bodybuilders have such small breasts because they have such low body fat