You know, it's funny you should mention that...My wife complains that she's putting on weight, though I can't really see it, and I myself am decidedly portly. Both of us are public-school graduates.
Our children, however, who have never attended a classroom school of any sort, are all rail-thin. (Well, except for the two-year-old, who is still a chubby baby.) My wife took our six-year-old for a checkup, and the doctor remarked on how he was only 25th percentile weight for his height. My wife said "Do you think he should be heavier?" The doctor said "OH no! I'm just glad to see a kid who's not grossly obese."
We eat pretty normally, I think, except that we don't buy a lot of prepared foods and it's rare to find chips, snack cakes, store cookies, or soda in our house. We eat a lot of meat and potatoes and homemade bread, and our share of baked goods certainly, but all made at home. Not light fare by any means...and yet my boys have washboard stomachs. My older daughter studies ballet, and so is rock-solid, but still not thick. Hmmm.