A Supreme Court justice Potter Stewart famously said of pornography that it was hard to define, "but I know it when I see it". The same thing can be said about beauty. There are so many different cultural opinions of beauty across the world. Piercings, rings around the neck, different skin colors, different hair colors, body types, and many other things are deemed attractive in different cultures.
For instance, in America being skinny has become analogous to being beautiful. A slim body is seen as beautiful in our western culture. However in many parts of Africa, larger women are seen as much more beautiful than slim women. In Mauritania, this even leads to force-feeding, and is a social issue similar to the epidemic of eating disorders in America such as bulimia and anorexia.
Why do different cultures find such opposing body types appealing? When you think about it from an evolutionary psychologists point of view, finding attractiveness in larger people makes sense. As mammals, we're made to survive, and to do that we must eat. The larger the person, the more well fed and healthier they are. In cultures where food has historically been scarce, large equated to wealth. The problem is, now that cultures are progressing to be more industrial and western, scarce food has become less of a problem and that is why obesity is becoming an issue in these places.
This explains why in America we don't view fat as beauty. Most people have enough to eat, so a large body doesn't equate to wealth. A skinny body, in turn, could equate to many things, such as motivation or health (good diet). Environmental factors have changed the overall view of beauty. It's fascinating how mass psychology can be impacted by factors such as these.