Big boobies😁

I know this is going to sound a little weird and funny or maybe stupid lol but does any of you know how to keep the size of your boobies as is when breastfeeding after you done breastfeeding and for them not to shrink again :( ? 
I am asking because my sister in law every pregnancy her boobs grow and grow and they don't go back to her size she was before pregnant, I know she does something but she doesn't want to tell me !
I have a set of twins and I had biiiiig boobs when my milk supply was over they shrunk even more than before pregnant.
Now i have a baby girl and I'm breastfeeding as well and have huge boobs but I know once I'm done breastfeeding the same thing is going to happend.
Any suggestions? Or tips 😁😁😁