Everybody in North America got fatter.

This may be especially true for N.A, but it is not uniquely true for N.A.. Go walk down the street in Germany, then watch footage of either Germany or the USA some time before 1980. Everyone is skinnier in the old footage. While the US may be the fattest of developed nations (I don't know), everyone is plumper than before. People used to be skinny pretty much across the board. I would have called myself fat when I was growing up, yet people tell me I look good for me age...

In fact, you would be incorrect to single out humans. "as the American people were getting fatter, so were America’s marmosets. As were laboratory macaques, chimpanzees, vervet monkeys and mice, as well as domestic dogs, domestic cats, and domestic and feral rats from both rural and urban areas."

https://aeon.co/essays/blaming-individuals-for-obesity-may-be-altogether-wrong