Oof. I don't want to sound like I'm defending the statement, even though I do like her as an actress and find her blog and vlog's adorable and fun. Also, I loved her in Nick & Norah and find her to be refreshingly geeky.
I do think that the comment is woefully out of context. We don't even know what the question actually was, what led up to it, etc. I'm not saying that makes it okay to snark on slim women, not at all. Just that there's a huge part of this that's missing. I'm not expecting that to matter or excuse it, by the way, just that I like to know more about what led up to a comment before freaking out.
One of things I do know is that she has lots of body issues herself, and has to constantly deal with weight comments and issues while auditioning for parts. She's body snarked a lot online, as well. And I doubt anyone would argue that Hollywood doesn't have the most realistic body ideals. There is also the reality that thinness is, currently, a privilege body type.
That, however, doesn't make body snarking okay. And it's always unfortunate when women do that to each other. I think it's so automatic a lot of the time that we don't know we're even doing it.