If you look at high school yearbook photos from the 1980s, almost all the people were thin. These are young persons that should have high metabolisms and high rates of activity, so you'd expect that. That thinness changed noticeably and rapidly through the 1990s. I went to high school in the 1990s and could see that shift very plainly during the decade.
All childhood age range obesity was typically about 5% until the early 1980s. Now the combined average is around 15%
From 1960 to 1980 adult obesity was typically 12% to 15%. Today that would nearly qualify as the thinnest country in the developed world (only Japan, South Korea and Singapore beat it now). From 1980 to 1990 it went up five or six points. By 2000 it had doubled (since the early 1980s) to ~30%. So approximately a doubling of the obesity rate in 15-20 years. Contrast 1980 US at ~15% obesity, with present day Denmark, Italy, Netherlands, Austria and Switzerland which are all considered fairly thin at ~20% obesity rates.