She's also a box office magnet. I doubt that Weber has any real say in the matter of casting - writers aren't generally given much, if any, any say in the matter. Whether she wants the gig after playing a somewhat similar character in Sky Captain is another matter. Waifs like Natalie Portman need not apply.
Waifs? She looks to have a good three or four stone on the rather skinny and ill looking Jolie.