The cup sizes varies with band sizes though, (which is easily seen in underwire bras if you comapre the size of teh wire), so if you loose band sizes but keep the same cup size your cups are smaller too.
I don't easily loose band sizes since I don't carry much fat on my rib cage and since I alredy wear one size smaller band than they say I "should" with my rib measurement, too keep it tight enough to hold my breasts, I will keep the same band size unless I get really obese.

/Eva