She's naturally very tiny. I've seen her in person - she's as small-framed as get out. Even in those Jennifer's Body pics where she was teeny skinny, her ribs don't show. She looks a little on the thin side in the pre-viz pictures, but not sickly. And this is coming from another skinny! I'm naturally thinner than she is - my bones stick out everywhere, and yes, I know how horrible it looks, but I can't help it.

It would actually take a good amount of work (eating like a pig, lifting serious weights) to get me to look different.
It probably takes some work for her to pack on the weight to look the way she did in TF1 and TF2. It's not really the most surprisingly thing in the world for some actresses to HAVE to gain weight for a role, you know.