I'm a healthy weight and I'm very active (gym, running, hiking etc) so I consider myself lucky to be fit and well.
It's just that I'm rarely ever full. I can always eat more. I can and have easily out-eat my DH who is three stone heavier and 6 inches taller than me. I don't seem to have that 'switch' that says yup, that's enough. Even if my stomach feels full, I could always keep going. Like there's another space the food isn't filling...
I eat a very balanced diet full of wholefoods, healthy good proteins/fats/carbs ratio. No junk, processed food or sugar. My diet is probably better than average (special interest in nutrition, ironically) with lots of variety, I cook from scratch and eat three meals a day, I drink lots of water and green tea too so it's not thirst.
I have to really watch my portion control and not snack so I don't put weight on.
I reckon I could do these super-food challenges if I wasn't worried about the calories! Maybe my stomach is supersize?!