It's something to do with the pressure on the joints, I believe. All I know is not only is it gross but it makes them look even stupider. Not just that it makes their appearance stupid, but it literally makes them look like a person of low intelligence, more so even than other fats. Like they're not smart enough to stand up correctly. I think knock knees are kind of gross on everyone (I have no real opinion on Anna Kendrick, though, aside from thinking she was cute--as in, her performance as the character, not necessarily physically--in the one movie I've seen her in, which was also cute IMO) but on fats they are especially repulsive. Too lazy and bloated to even support their weight properly on their legs, they have to smush their knees together like the base of a house of cards that can't balance without that other side providing stability. I imagine if they tried to stand on one leg they'd just topple right over as soon as the foot left the ground, since their bodies seem to be upright thanks only to the principle of opposing force. I can't imagine standing still and having my knees touch while my feet are eighteen inches apart. I just tried to do it and it was painful and awkward, and looked ridiculous. I didn't even try to take a step; I don't think I could actually manage it.