I was the same at 16 - it was a combination of control (the first time I had my own cash and could choose what to spend it on,including limitless 'treats') and unresolved hunger.
The control thing (I think) was normal (I watched all my friends do the same and go on a year long binge of coke, chocolate, kebabs, and Pringles once they got Uni) but the hunger was something else.
Teenage boys in my experience are allowed to be hungry and eat to satisfy it - large steaks, whole chickens, bowlfuls of nutritious food but as a girl this would have got me looks and comments from my peer group. So I drank diet soda, ate diet yogurt, had smaller portions of the filling stuff (meat, fish, chicken, eggs, cheese) in public and then fed the hunger when nobody was looking. Feeding it on the good stuff would have required preparation (giving time for people to notice and comment) whereas chocolate could be easily smuggled.
Of course my parents noticed the junk consumption and so started to comment and look out for my smuggling in chocolate and sweets so I moved to fast food whenever possible - no preparation, instantly consumed and gone without a trace and, sadly, addictive. I got fat.
I starved myself when I got to 18 (I still remember it well 20 years later - an egg for breakfast, nothing for lunch, and a half can of tuna in brine with spring onions and half a chopped egg on a bowl of lettuce leaves every night combined with a lot of running) and made everyone happy. No more comments - I was thin and 'eating healthily' now and while my parents had been eagle eyed on my eating junk at 16 they magically didn't notice my absent lunches.
I continued to yo yo for the next 5 years, binging and starving alternately. Then something changed, I tried one if the many low carb high protein diets around (eating like a man, Henry VIII, to be precise) and noticed that for the first time in my life I didn't feel hungry and my weight stabilised. I realised what I understood until then had clearly been wrong, gave myself a break and ate proper 'manly' food instead of the diet alternatives and have never looked back. I do still get comments though: 'surely you can't eat all that and stay slim','are you bulimic or something?'.
Apologies for the epic ness of my post, your OP clearly struck a chord with me. Just suggesting that it might be a physical issue, hunger, particularly that she has already been on a diet by 16, in addition to a love for (addictive) junk food.
Ps. Exercise is another way of attracting unwanted comments - I run three times a week now and still get the odd one but it was much worse as a teen. I didn't run then because of it.