This must be really hard for you, since I'm sure you know that your daughter is hurting her health by continuing to gain weight, and putting herself in jeopardy of getting diabetes (which has increased in near epidemic proportions among teenagers and young adults in the last few years). And I know you want to help her in any way you can. I hope you can find some answers on this forum.
I too was a "chunky" teenager and my mother never seemed to have a problem with her weight (it sounds like you don't either). She always tried to shame me into losing weight, thinking this would help motivate me. But instead, it just lowered my self-esteem more (I already felt bad enough because I was overweight), and made me depressed, which ultimately caused me to eat even more. As you probably know, there are many reasons why people overeat. Sometimes it's just out of boredom, but other times it's because of depression, etc. For years, I used eating as a pacifier, whenever I was nervous or depressed. But instead of being sympathetic and trying to understand that I had a problem, my mother was always very critical. So for years I struggled alone with my overeating problem. I'm 51 now and finally feel that I can deal with this in a sensible way.
For what it's worth, my advice would be to
encourage your daughter by going on a diet with her. This might sound crazy, but even though you don't need to lose any weight, almost all of us could stand to eat healthier. I've had the best success losing weight with a modified low carbohydrate diet ("The Carbohydrate Addict's Diet" is the name of the book I use). It's fairly easy to follow too. Once a day you can eat whatever you like, as long as the carbohydrates (cakes, candy, pasta, potatoes, bread, etc.) you eat are in equal portions with the low carbohydrate veggies (broccoli, cauliflower, cabbage, celery, lettuce, etc.) and meat that you eat. That way your daughter won't feel like she's being cheated out of all the foods she loves. The other two meals a day would consist of meat, cheese or eggs and a low carb veg. People who don't need to lose weight will maintain their weight on this diet, but overweight people will lose. And the best part is that most people actually feel healthier too, as this diet will help stabilize blood sugar, control blood pressure, etc. You need to read the book to find out all the benefits (it's got some interesting stories in it, so I did not find it boring at all).
Anyway, I wish you all the best in helping your daughter.
Karen