Why do skinny girls try to force their body image on other people?

I kind of find it annoying that some of them hate on girls who are average weight or just a little chubby. They make them feel bad about their weight cause they are brainwashed by the media that being underweight is the most attractive and healthy choice. When in reality, girls are suppose to maintain the hour glass shape and stay at the right healthy weight(not too fat and not too skinny). I don't want a rectangular figure. I have no problem with those girls but they can live their own lifestyles and not try to insult other girls for not being as skinny as they are. No offence intended.