i dont think its an issue of size (ie that fat people shouldnt wear certain things) its more about wearing what suits you. anybody, of any size, height, colour, sex, whatever, who dresses in clothes which are unflattering to their personal shape and colouring is going to look bad. you cant get around that. sorry to be blunt but a size 18 person of either sex is most likely not going to look great in minishorts, and will probably be judged for wearing them. likewise a size 6 person in the same shorts could look equally bad. and im not saying that larger people should not wear short summery clothing, im saying they should do it tastefully and perhaps realise (and this goes for people of any weight) that there are options other than miniskirts etc which keep you cool in summer. eg - if you are very large around the bum, a flowy skirt might be better for flattering the figure than a mini. it's heat reducing, it looks decent. the person is less likely to be whispered about/pointed at etc. everybody wins.
ok, having read over that i dont know if i am making sense. so the basic point of this is: dress to suit your body shape, dont try to suit your body shape to ill fitting clothes, it looks bad. if it fits and flatters, it's good, no matter what it is. if it makes you look like a jerry springer regular or look 10 sizes bigger then its not so good. WEAR WHAT SUITS YOU.