I disagree with your premise..... I remember my grandmother was overweight, and she died in the late 1980's..... my memory of my mother is that she was always overweight.  Really she was not overweight when she got married, but my memories is that she had always been overweight.  I can recall many, many people in my lifetime who were overweight.  

Now, if you want to talk about school kids, I think there are more school kids today that are overweight than years ago.  Still, even when I was in school - in the 1960's-1970's - there was a wide range weight in a class of kids.  Personally, I was a bit on the heavier side when I was in grade and high school. I probably would have been even heavier if I hadn't been doing physical work on the farm during my growing up years.  

When I was born, I was considered to be a big baby.  My birth weight was 7 lbs 6-7 oz. In that day, that was considered heavy.  I don't keep track but I suspect that would be considered a normal birth weight today.  I have often wondered over the years, if better nutrition during a woman's pregnancy is contributing to heavier baby's being born, and if that is contributing to more obese people in today's society - both kids and adults. It's a question I have often wondered about.  

edit: Apparently there was a study done to look at historical birth weights.   Scroll down to figure 1.  Apparently, prior to 1970, babies had smaller birth weight, but gained more weight in the first 12 months, compared to years since 1970.