I have smaller breasts too and it's hard to find lingerie that fits right and is flattering to my bust. A review by novanilla she lists a con for this as "halter and triangular top will make small breasts flat and large breasts pop out, no support".
what size is you breast and around. To be honest (wife here) I wear a 36D and mine are a little saggy but I have had 2 kids and it works well with mine. The string does offer a little support and should work fine with a larger A cup on up I feel as the bigger you are this babydoll doesn't work to fine it kind of smushes my breast sorry I didn't see this earlier.
and as you can see from the pics in my review it makes my breast look flat and there is no popping out I really think this was made for a A to small C cup.
I'm an A cup, and if I was wearing something like this, I would wear those silicon self-adhesive boobies underneath Otherwise, because of the cut of this dress, it would make mine completely invisible!