Actually the correlation between fatness as a child and fatness as an adult is about zero. You are not more likely to be fat as an adult if you are fat as a child.
That being said, no matter what you weigh as a child you are probably going to be chubby as an adult. That’s the way it is now.
To see what it used to be like go to youtube and look at the old videos of Dick Clark’s American Bandstand and the old Soul Train videos. Compare the BMI of those kids with the average high school student today. Big difference. It is amazing how thin teens were 40 years ago.
I weighed 99 pounds when I got pregnant, I ate like a horse.
I weighed 164 pounds when I gave birth.
My kid was 8.1 pounds at birth.
He was a beer belly dude til his 15th year,
He is now 5’ 10” and weighs 129, his weight has not changed in two years and he’s built just like me, and my Mom and Dad before me.
Go figure.
I have noticed that as well. There used to be all these slender young people. Getting plump was for the middle aged. Even in the 1960s, our school photos show virtually no fat kids.
Also, on a historical film, there was an early film from New York in the 1900s. The crowds walking in the streets looked so slim, that the view of the film seemed distored. But it was not, as one could see by looking at the buildings - they were the same as now.