Compulsive overeater without an "off switch" here. I think your daughter is coping with stress through eating. Does she sneak or steal food more frequently during times of stress? When there is stress and she makes "bad" choices, do you withdraw emotionally or criticize? Just questions, please don't take offense. Just saying that in my case (which might not even be remotely the same as the situation with your daughter) these issues all exacerbated the problem.
And again, not to offend, but I do have a few suggestions. Keep in mind that I don't know your kid at all, so these things might not be relevant. I would suggest that you seek counseling for her. She has some underlying issues that need to be addressed and therapy is a "safe" place to do that. A good therapist will offer insight to her problems and suggest alternatives for the poor eating habits. Secondly, I would address the way that you are handling her sneakery. And be honest with yourself. If you come across as critical or less than supportive, understand that some subtle changes in your behavior or seeking counseling for yourself might be beneficial. And finally, I would remove poor quality food from your home if it upsets you that she desires it. If she is a true overeater, like I am, the urge is almost beyond control. We'll seek out any food item that "banks" calories. Call it a mis-wired evolutionary drive for calorie storage, but we are motivated towards high value treats. If you know she will eat them, and you expect her to have self control where she has proven none exists, you will set her up for failure.
I hope that I don't come across as harsh. I don't intend to. And I am certainly not placing blame on your or your environment, I hope that you understand that. But as parents, we have to manage our situations in ways that hopefully facilitate the best outcome. And that is what I am trying to suggest.