No-one encourages obesity! Let's remove the argument from weight. Would you be inclined to dress up if someone told you your dress-sense stinks? No, you would tell them to fuck off and continue to wear what you want. But if someone commented that you dress well you would take a little extra care with the outfits you choose. I would never tell a fat person that they have a gorgeous figure, it would be a lie. But I would tell them if their dress is flattering, or if I like the colour, for example, sincere compliments that encourage them to feel good about how they look. Part of the whole package of wanting to look good will include losing weight, which I would then compliment.