I don’t know about this. I think it might be possible to eat only “healthy food” and still be overweight if the portion sizes are just too big on a regular basis.
My DC definitely eat some junk food, they also have sweets and chocolate a few times a week. They are really slim and so am and I and DH.
But we don’t eat big portions at all. Some of the overweight children I know come from families where the family sit down together every evening and the mum cooks, which should be a good thing. But I think a traditional “meat and veg” dinner everyday does add up. Some people also have pudding after each meal, as if it’s standard!
We are often rushing about and might have scrambled eggs with pitta bread or beans on toast, or pesto pasta - we only sit down as a family for a “proper” meal once a week. So, combined with DC having McDonalds once a week on the way back from an sports club, probably not as healthy as we could or should be.
Maybe “healthy” is mixed up with home cooked food now in peoples mind as well. Jamie Oliver talks a lot about healthy eating, but if I followed some of his recipes and portion sizes every day I think I’d be pretty big.
My friend - who has very overweight DC and is big herself - cooks from scratch often, which is seen as a healthy thing (and she describes the DC’s diet as healthy) but she might make a home cooked lasagne and then adds garlic bread and chips as well.
To me, lasagna seems such a big meal, that a bit of salad on the side is quite enough, but if having sides is what people are used to then they might not notice that they are packing in additional calories. Even a “healthy” pudding like Greek yogurt with banana is also quite high in calories.