Out of curiosity, and this is a very genuine want to really know why:

What is it about the lean girl that causes you all to see her as being anorexic or having an eating disorder?  I honestly can't see any indicators of that on her body, not anywhere.  If she was undereating or anorexic you'd see her rib cage.  There's not a rib in sight, even her hip bones have flesh around the crest, they aren't popping out of her body like they would on an anorexic girl.  Her face looks perfectly normal as well, not withdrawn.

She just looks naturally lean but shapely to me, a girl who is lucky enough to have a good metabolism and have a nice pair of hips as well.

When moriador posted I sort of giggled and thought to myself, well, that could be the classic thing you see among almost all women, perhaps secretly she's envious of her figure.  But a lot of the comments here are from men so that puts a totally different angle on it.  Granted, she's not the babe-type figure that kim is, she's just "statuesque". 

If you had to be perfectly serious and analytical about it, what is it about her that stands out the most to you when you first gance at her; what's the main feature that makes everyone see her like that?

If it's just the overall impression of her, don't forget she's on tip-toe, a classic trick to make the legs look longer.  Women generally have hips with a notable dip around the waist, she definitely has that as well, by the bucket-load.  Her breasts are big enough that the sides protrude her ribcage even.

She's a stunning example of the female form in my book, and very statueque.  Hell, if she was a 3D mesh and the whole scene was rendered in white with AO, she'd look like the perfect statue!