I have seen women "dry up", so to speak, after breastfeeding. Some women actually gain more cup size, but I have seen breasts feeding work against the aesthetics of the body by the breasts looking like flabby pieces of skin hanging off the chest. Who is at risk? I am only a B cup with a perky cone shape, am I at risk? Should smaller breasted people only breastfeed for a certain number of months?
Are you currently breastfeeding? How old is your baby?
It is actually pregnancy which causes a woman's breast tissue to change (and sometimes become more 'saggy') rather than breastfeeding. As you say, breastfeeding can even lead to fuller breasts. It is important to wear a suitable feeding bra when you are breastfeeding (in the day and night). You can also do muscle toning exercises to help with your breast appearance (swimming is an excellent exercise for this) and moisturise daily. Eating a healthy, balanced diet will also help.
Above all, smaller breasted women should definitely not reduce the duration they breastfeed for. Breastfeeding has hugely significant advantages for baby and mother. It reduces both a baby and mother's likelihood of suffering from certain types of cancer. It contains stem cells. Breastmilk has brain developmental advantages compared to formula milk and it is gently on a baby's digestive system. It reduces the likelihood of a child going on to develop diabetes and numerous other health issues in later life. Breastfeeding is 'normal feeding' and, unfortunately, formula milk is vastly inferior to breastmilk's complex yet delicate make-up.
The World Health Organisation state that babies should be breastfed exclusively (i.e. only have breastmilk) in the first 6 months of life and then be breastfed alongside the introduction of solid food from 6 months of age until 2 years of age and beyond. This is the same regardless of a woman's breast size. There's also no truth in the myth that smaller breasted women can't produce as much breastmilk.
I hope this helps. Breastmilk really is wonderful stuff!