[fredrikholm]

Selective breeding of potatoes started in the beginning of the 20th century; growing potatoes long to be very large increased the risk of crop failure due to blight (and other problems).

Given that, combined with the lack of modern farming equipment/resources, I think it's fair to say that the average farmer didn't come close to eating 6000 calories a day. Even at very high energy expenditure levels, that would bump you way over a 100kg bodyweight.