This was discussed at great length earlier this year, with many people baring their own, as well as familial breasts and histories.  I think the consensus was it has a lot to do with genetics and little to do with lactation -- some people will sag, and some won't, some will return to almost prelactational status, some won't.   Just as some non BF, and even never pregnant will retain their shapes, and some won't.

Chanita, San Francisco