As someone who was naturally very slim for a long time (alas no longer!) and was quite wrongly deemed anorexic by a so-called friend who spread it around for months, I do really dislike the tendency for labelling the very slim like this - especially when it's someone you've never even met.
Some people just are naturally slim. Some people are not and never have been big eaters, though given that so many people are overweight or obese nowadays, it seems quite common for 'anorexic' to be chucked at anyone who's never had that problem at all.
I do often have to conclude that it's jealousy on the part of fatter people, who can't imagine that anyone else genuinely isn't interested in lots of cakes/biscuits/other fattening foods.
Such was certainly the case with the 'friend' who spread the rumour about me.