Men are usually more attracted to slim girls with defined well-proportioned figures and ample breasts. This attraction is programmed into us biologically. For men, these features are a subconcious sign of good health and a good indicator of fertility.
Fat girls, on the other hand, may say to us: she is unhealthy, she has a bad lifestyle, she is more likely to suffer serious health issues as a consequence of her obesity, she may die young, she may not be (so) fertile, she may pass on her genetic obesity to my children.
Sorry to be so blunt but that's the world of sexual attraction and mate selection.