Prolactin is a breastfeeding hormone, that's why there is fluid coming out of your nipples.
It should make your breasts larger, but its effects stop working, because your body gets insensitive to prolactin's effects. Once it stops working, increasing prolactin won't help. If you're not getting NBE effects, then it already stopped working.
"and when they reduce the prolactin somehow, will my breasts change?"
Perhaps they'll stay the same or slowly shrink over time, but depending on other hormones your breast size can change in either direction. Without NBE, not by much.
It should make your breasts larger, but its effects stop working, because your body gets insensitive to prolactin's effects. Once it stops working, increasing prolactin won't help. If you're not getting NBE effects, then it already stopped working.
"and when they reduce the prolactin somehow, will my breasts change?"
Perhaps they'll stay the same or slowly shrink over time, but depending on other hormones your breast size can change in either direction. Without NBE, not by much.