If as a result of diet and exercise one's breast size is reduced, will
her breast size go back to the pre-weight loss size (cup size) if she
gains weight?

puhahahaha....
i'd really like to see if someone put in time and research money to
study this lol...
I think if you do get an answer, it'll be an opinion of some would be
expert that hasn't been proven with perhaps some anecdotal evidences
at best...

I found sites that confirmed that dieting may reduce breast size, and
one site mentioning increased size with added weight (which is obvious
in most cases if the weight gain is is too obvious).  Back to pre-diet
size??

Where are all those gals with experience with yoyo dieting?