Your self esteem issues or hers ?
Talk to your ex husband and explain the situation to him. If he loves ypur daughter like you do he will stop buying her junk food. Best of luck.
Some children or teenagers generally just have a large fr<x>ame, if this is the case there isnt much you can really do, it may be difficult for her to do this but it may be important that she stays away from fast food because if it is often then most likely this will be the case. If it is really getting to her then go to the GP's, they wont offer drugs but advice that can be very benificial.
Trust me, i completley understand where you are coming from. Well have you thought about talking to you ex partner about this? Are you scared more of her feeling insecure or her getting an eating disorder? More then anything its important that you try your best to make her feel happy with herself, new hairstyle maybe dye her hair new clothes all of this to really try boost her confidence, this isnt usually the way to go but with this society, im a teenage girl myself and i know how low self esteems are there horrible, but when all this happens it makes you feel amazing, this is important for your daughter, otherwise she may start turning to more fatty foods for comfort as a lot of teenagers do, id suggest a confidence boost! I really hope everything works!!
She's 13; she'll probably grow out of it. In any event, there are worse things in the world.
Have a talk to her dad - if hes the sort that will listen. 0therwise I'd say go the confiedence boosting way as TinkerbellaMay suggested.