The super-slim 'Atonement' star insists most of the women she auditions against are far smaller and skinnier than she is.
She said: "Weight is a big issue in Hollywood because I'm twice the size - height and everything else - of most of the girls who are going in to see the director for a part. When you realise that I am, at my size, one of the largest actresses there, you start to think, 'I don't think it'd be healthy for me to stay here much longer.' "
The 22-year-old actress has been criticised in the past because of her weight, and accused of having an eating disorder.
She was awarded £3,000 damages earlier this year after a British national newspaper suggested she was anorexic.