There have always been fat people. But they weren’t as common in those years. The fact that most had to do somewhat more physical labor probably played a part; but I think the main culprit today is the overly processed diet.
I remember one fat girl in Elementary School, same girl through HS, about 200lbs. There 3 really fat A-A in HS, 2 males, one over 325, the other about 230. The female was skinny by todays “Honkers”, probably 230 or so.
My sister was teased for being fat, she weight about 135 in HS. Girls were REALLY skinny comparatively.