I saw the movie and when I first saw her tiny waist my reaction was "woahhh, wow", but then I saw her arms and puffy face which completely killed my previous regard. I mean, it's really weird, her different body parts don't even match together. Maybe she wore some kind of corset.... i wonder how she can breathe in it.
I worked tech on a production of Les Mis. None of the main girls, Eponine, Cossette, Fantine etc wore corsets because it is constrictive to singing apparently (no clue myself, couldn't carry a not if I tried). Obviously, since this a film though, that rule is not applicable.
She is for sure wearing a corset, just judging from the way her chest looks. Her waist does look nice, which is the reason why corsets rock, but honestly, they kind of puff a lot of other things out, i.e. arms, hips, boobs, in a way that isn't so attractive. To rock a corset, you have to be seriously seriously tight or skinny.