>Clearly 20-30 years is not enough for genetics to play in

Unless it's some sort of epigenetic factors changing genetic expression?

It's a possibility considering our animals are also getting fatter [1], including lab animals, whose diet is controlled and has not changed.

[1] https://www.theatlantic.com/national/archive/2013/10/its-not...

Could be more cars and computers that pushed sedentary life on people or plenty of other things too: more effective sugary food advertising on better screens, food producers figuring out that sugar is addictive and starting to put it into everything, or all of the above.