The reason people hurt other people by making them feel inadequate and inferior is that they want to try to escape the feeling that they are inadequate and inferior, so they try to project it onto others. The problem was never actually your size or her size, it was her insecurities and the way you were letting them destroy your own self-esteem.
You did the right thing by walking away, she was sucking the happy out of you. She's an insecure cheater and you can do much better. Make sure you take the time to heal and to understand how to avoid getting involved with someone with a destructive personality, before you begin another relationship.