It gives you a big pain in your stomach, for one thing. It also repels decent men and attracts controlling jerks like a magnet.
Stop worrying about a guy's old girlfriends, the beautiful oncologist who takes care of his father, and anyone happens to be thinner than you. Never compare yourself to anybody but yourself.
When you're with a man, your attitude has to be, "I'm a great girl, and you'd be lucky to have me."