Nailed it, but there's more. Those other groups don't take up 2-3 seats on public transit, or look physically grotesque.  Also, the others don't typically let the addiction interfere with their normal day-to-day life. When they do, we call them addicts. Fats, by simply being fat, have let food force them into a dull, sedentary lifestyle punctuated only by brief moments of food-induced pleasure.