It combined with a corruption of HAES: Originally it was "Health At Every Size", as in, an encouraging message that you can always improve your health no matter how big you are (which a lot of people need because when you start exercising you can end up gaining weight before losing anything, because you're building muscle faster than you're losing fat). But at some point it became understood as "Healthy At Every Size" which people took to mean your size doesn't impact your health.