Matt Yglesias makes a catch which I didn't see in the original NY Times article on how our physical work is declining at the same time our weight is climbing. Specifically: "Running a pre-mechanization farm is hard work"
In the original article the focus is on the changes since 1960, particularly the decline of manufacturing jobs. But the same probably applies for farming. Our tractor, a small John Deere, didn't have power steering so you definitely could use some calories just driving the darn thing. Not that I've been on a tractor since, but John Phipps would lead me to believe that all tractors are air conditioned with power steering and a sound system. And certainly a lot of the farmers you see on TV have good sized bellies. I remember one barrel-chested farmer from my youth, but mostly they were muscular but not fat.