Why do people always have such a downer on thin women ?

I mean, there always seems to be too much comment on this women. Like the latest one to ' benefit ' from such attention is the newly minted Duchess of Cambridge. Oooh, coo the press, Kate is sooo thin now blah blah blah. But the other night I was watching an athlete called Mo Farrah winning a 5000 meters race and that guy is like a stick insect, he just has no fat on him. But no one commented on his thinness as he sped effortlessly to victory. Strange. Surely people should have a downer on the much greater problem of obesity