Oh, I am very sorry to hear this OP.
I am a fussy eater and have a small frame and a small appetite. I like being size 8 and sometimes I dip into size 6. I have also dipped into size 4. I exercise a lot too as a thoroughly enjoyable hobby.
I am self-aware though, which is a saving grace. I am HUGELY disciplined and if I said I won't snack, I wont eat oil etc etc I can stick with it till cows come home. The discipline has earned me over a million in work- yeah, sheer, natural discipline and determination.
Because of my self-awareness, when I am at size 6 which is easier for me to maintain than size 8, my self awareness kicks in and I add a few more healthy foods item that I enjoy. Hence I am always size 8 or occasionally size 6 which is usually unintentional- when I am at 8, it just falls off further to 6 and have to rein it a bit back.
What I am saying is, I am in my 40s but have always understood that, one mistake (too much control etc etc) it is very easy for me to see how it is easy for people to develop anorexia and have food issues. Perhaps, after people have been too fat and in the process of losing weight they get the good feeling and realise weight is the only thing they can control in their lives so they stick with it. So, as one poster asked, other than being surprised OP, check what's going on in your DD's life and how she can improve those areas other than punishing herself over food and being only in control of her eating to feel good about her life.
My niece is overweight- knowing the above- I try never to push her, but just support her with her ongoing plan which never really yields weight-loss results. But I would rather have my niece trying than becoming anorexic or bulimic in the process.