I've got a large chest for my size... petite frame (5'5" about 140 lbs) but a size 34 / 36 DD.  Since this was strapless, I went a size smaller like they say.  I've used it a few times, and it'll stay in place for about an hour or so, but definitely not much longer.

Luckily, you can utilize straps, so I still use it that way, but I had specifically bought this to be strapless and am a little bummed I can't rely on it in the way I'd like.

However, it is comfortable as a "regular" bra, so it's not a total loss!  :)

Oh, and be aware... if you want to use this for a dress that has a low neckline, be wary.  The middle of the bra comes up VERY high, and can be seen even with shallow Vs.