Mine are a lot saggier than pre pg as well.
I put this down to the weight gain during pg, which stretched the skin. And then when my milk came in they were stretched to mammoth proportions that they had never been before - even though I didn't succeed at BF for very long.
I have also heard that breast fat is replaced by more milk ducts when BF, which dont return to fat once BF is over - hence boobs also being more deflated/saggy, however I dont know if that last one is true or not?