Think there are many stereotypes and assumptions about what is healthy, about what is unhealthy, about what is normal eatind and activity, and why one would choose one way of eating and behavior over another, and of course the assumption that you can tell how a person eats and lives just by looking at that person.
I gained most of my weight on what is usually considered healthy food in unhealthy portions. I first became fat a few months before starting kindergarten - around the time I was allowed to cross the street with my little brother to vistit our grandparents after dinner (we thought we were hot stuff because we thought we were crossing the street by ouselves, even though Mom would walk with us until she could see our grandparents' house and could see there were no cars coming. If there were she'd shout for us to wait for the car to pass).
Our grandparents ate dinner later (or maybe grandma held dinner until we arrived), but at any rate, the end result was my brother and I eating two full healthy dinners. My brother, who had a problem keeping weight on until well into adulthood stayed thin (he was also what today would be called hyperactive - my parents and grandparents were always trying to get him to eat more, but the more he ate, the more energy he 'd have and he'd burn it all off. By highschool he was eating twice what I ate, and maintaining a weight about half mine).
All my life people have remarked how healthfully I ate, and were either wondered how I could be so overweight, often assuming I ate huge amounts of junk in secret.
I knew why I was overweight -- because I always felt hungry, even when stuffed and I didn't get much exercise. I didn't realize it until adulthood, but my mother discouraged me from most physical activity either because of her own fears, or because such activities were "unfeminine." One year she pulled me out of tennis lessons because she heard kids making fun of me. I didn't care, but my mother did. She'd also tell me how "silly" I looked, or how fat people just didn't do certain things because they caused sweating and stinking.
I know today that sounds terrible, but 40 years ago there were a lot more social taboos regarding appropriate behavior. I knew before the age of 10, that I was supposed to wear dark (slimming) colors and shouldn't wear stripes or sleeveless tops. There were so many fat-girl rules (there still are, but rule-breaking is more acceptable).
Back to the original topic though, I've had to learn that healthy eating is about much more than choosing specific foods. For myself, I can choose (what seems to me) very tiny portions of moderately healthy, and mid-range calorie food, or I can eat larger portions of very low calorie foods. Usually, I'd rather have tons of veggies than the same calorie potion of grains or beans (even healthy starches tend to increase my appetite).
For some reason we humans tend to gravitate towards cure-all and one-size-fits-all thinking. And when it comes to food and exercise habits, we've come to believe that any deviation from the "normal" only makes sense in the scheme of fixing something that's broke - weight loss, allergies, diabetes, heart disease.... we're not too keen apparently on preventing illness only treating it (and even then only when it's convenient).