People lust for the schadenfreude, particularly when it's visited upon those who possess things the people covet. If we cannot acquire what we covet, we denounce it, thus reinforcing our self-affirming bias against the very thing we covet. We use this practice to reconcile the juxtaposition of lusting after something we simply cannot have. To do otherwise leads to two inevitabilities: a lifetime of misery, or an all-encompassing passionate pursuit of thing we covet (which is unlikely).
Meanwhile, by bestowing well-wishes upon those we view as sorrier than us, we feel an uplifting sense of well-being that helps us tap into our general desire to show some sense of empathy and altruism to those misfortunate few. We do this as it is obvious that paying compliments to such a sorry person couldn't possibly lift them up to our perceived social status, and in fact, even elevates our social status by demonstrating our ability to function in a society that value altruism to those less fortunate. However, most draw the line at paying a harmless, throw-away compliment. When push comes to shove, most humans would not go out of their way to help the unfortunate fattie, nor would they choose to be seen in public with her or to be viewed as being associated with her.
By way of comparison, the anonymity of the internet allows folks to denounce the very things they covet in private, to confirm their bias. But in person, most humans would, if given the chance, choose to let the fit people into their circle of influence. As surely, fit attractive people are far more useful to one's social standing and the perception of their net worth, than a fat woman.
Humans are interesting and unique, but as a larger sample group, we typically behave in the predictable fashion you would expect from the slack jawed mouth breathing masses we so vehemently rail against in an effort to espouse our individualistic nature.
