Part of the difficulty is that it's so easy to judge people for being fat (and assume they overeat) when we don't know why they're overweight.
I became morbidly obese while taking medication for a health problem for several years. It's a medication that is notorious for causing weight gain. When I finally stopped taking it, I started losing weight. It was neither safe nor convenient to restrict my calorie intake to less than 2500 calories a day (because I was so obese), and so no doubt I looked (to strangers) as if I was overeating, and yet I was losing weight. I'm not back to my pre-medication weight yet, so I'll be 'overeating' for quite a while.
My daughter is very overweight. She has a serious health condition that contributes to her obesity. There is no way that anyone would guess by looking at her. Many people simply see a fat girl (and probably think it's a shame, but no surprise since her mother is fat too - no one would know there are different reasons for our weight).
People who think that medical reasons for obesity represent only a very small percentage are simply wrong; there are a great many health conditions that can play a role in obesity, and most of these are invisible. Many of them are - like obesity - increasing in prevalence, such as diabetes, autism spectrum disorder, depression etc.
So if anyone is 'not surprised' that my kid is fat, they're probably completely unaware that it has little to do with what she eats at school.