My guess would be that your ancestors lived a modestly different lifestyle. I suspect they weren't spending three hours a day commuting by auto, to and from their completely sedentary jobs, and then slumping in front of Iron Chef, The Biggest Loser, and Serious Eats at night. Your great-grandmother was probably at home cooking, rather than trapped in the office all day. I'm pretty darn certain that there weren't hyper-palatable fast foods on every street corner and eight aisles of hyper-palatable treats in every supermarket. Or any supermarkets.
Of course, it could also have been the government's anodyne, common-sense recommendations, too.