Maybe it's a perception/personal taste thing, because "starving/impoverished" was very much what I got from her appearance. Granted, she looked more like she was recently impoverished than I think the movie had intended, but ultimately it still seemed to convey that to me. She was hungry, ergo she was tying that waistband really, really tightly so she wouldn't feel that hunger as much as if she loosened it.

It's something I've done myself when I was hungry and food wasn't in the immediate vicinity - in my case, it was grab a scarf, wrap it around your waist, and pull as tightly as possible. Boom, it's like you were never hungry. The downside is, of course, that it distorts your figure, and of course it can rearrange your internal organs if you wear it like that long enough. But outside of that, if you are hungry and are unlikely to be able to eat any time soon, it is a very practical thing to do.