Yep, my 70s elementary school class photos show the same thing - no hamplanets, few chunky kids. I remember my high school measurements because that’s when I learned to sew.  My 24” waist & 34” hips meant I wore a 4 in misses, 5 in juniors. I thought of myself as “medium” because there were plenty of girls skinnier than me. Both the explosion of young fatties and the vanity sizing are within the last 20-25 years.