My 14yo was seriously overweight between about the ages of 10 and 13.
We do eat healthily as a family, but obviously, there is always the option of eating too much. Also, when she started at senior school, she had much more autonomy in what she ate at the school canteen. (Although, they use parentpay, so I could check what she was eating online.)
Anyway, we got to the stage where I was really worried. We started weighing once a week, to check her BMI, which was very high, and showed her to be obese. (The point of the weekly weighing was for her to be able to see it gradually fall.) We discussed healthy eating, and what snacks and breakfast foods she might like me to buy (I started getting hot smoked salmon to eat on bagels or toast and avocados, and so on. High in energy, but not sugary.) I said we needed to sort it out, and that I was worried maybe she had an eating disorder, and that was nothing to be ashamed of, and we could go to the doctor to get help.
In the event, she has managed to bring it under control, herself. I think this is partly due to falling out with a "so-called-friend" who was bullying her in that "mean girls" way of blanking etc.
So basically, I gave her some support to sort out the problem, but, she needed to take responsibility for it, herself.
Now, at 14, she is slim, but not skinny. She eats healthily, but not obsessively so, and I don't even bother to check what she is eating online, and as far as I am aware, she hasn't weighed herself for months and months. (I asked if she wanted to check her BMI, to see it had gone down into the green range, and she wasn't interested, which I thought was very healthy. She could see that she wasn't overweight, anymore. She didn't need to prove it to herself with a number.)
I don't know if this helps, at all. I do have a (nearly) 16yo, so I know dealing with them can be harder than a younger child, who may be more receptive to your point of view.