Some women lose breast mass when they lose weight overall, but others do not. I have a lot of "dense" tissue, i.e., glandular and ductal tissue, like Jamie D mentioned, so when I lose weight, I stay the same amount of tissue but look bigger. The next several months I will probably be losing some weight, so my band size will decrease one or two sizes, perhaps, but I will have to go up a cup size, because my breast tissue will not be getting smaller.
And you can see this when you change bra size: down a band size, go up a cup size, so if you used to wear 40C, try 38D. E.g.,
As Sybil mentions, you can try binding at work, which is what I did until just over a year ago. As I got larger recently, it became impractical, but my boss was great about it so now I am transitioned at work, I wear my bra openly now, 24/7, THANK GOD! (Binding was really painful.) I never thought I would transition at work, but as it turned out, it became a necessity.
What I also did for many years was CAMOUFLAGE my shape. Ever notice women wearing the blouse over t-shirt like this tv commercial?

(Sunsetter awnings advertised every five minutes where I live!) This actress is not that big. But women love this clothing solution in various forms, even if they are not very large -- just instinctively protecting our breasts from unwanted attention.
So figure out what you want to do, but in the end, it is healthiest to be proud of your shape and do not hide it!