My difficult child was considered obese when she came to live with us, and she ate constantly as a way to soothe herself when she was with biomom- binge eating.
I only kept healthy snacks in the house, so when she said she was hungry and wanted something to eat, the only option was something healthy. Since she was a carb junkie, she would rarely want that.
I also started walking with her daily, about 3 miles a day. I'd put my oldest in the baby carriage and head for town, promising difficult child a snack of something when we arrived. The snack itself motivated her to do the walk, which eventually led to her losing a great deal of the weight until she went back with biomom and the binge eating began again.
There have been studies recently that pointed to the fact that some obese people do not ever feel full.
Just as an aside, my oldest has been diagnosed with adhd and is on medications. Eats like a horse and looks like a string bean.