The pregnancy hormones are the culprit. Prolactin and Oxytocin aren't 'deflating hormones', but relaxin, hCG, estrogen, etc are.
Not directed at you... but I really with the misinformation that breastfeeding causes sagging breasts would just go the way of the Dodo (extinct animal). It causes many women to be self-conscious and less likely to nurse because of their vanity - when it is really just the pregnancy hormones.
It is very rare for a woman to have the same breasts after a pregnancy, just as their body will always be a little different.
Anyways - why not try those inserts that go into the bra cups? I wouldn't get implants either - especially if you want more children. You can always get pregnant again - this time breastfeed longer so your breasts stay fuller longer. Once the milk isn't being produced anymore... your breasts go back to pregnancy sized.