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.
If your kids sit and study five hours a day that is at least three hours less then most public school students when you add in the time they spend on the bus and homework. If they are just mildly active during those hours they are burning off a lot of calories and building muscle that in turn burns more calories and so fourth.
The problem is not just what we eat, although I would bet your kids don't often eat a bowl of chili-cheese fries for lunch, it is that we aren't moving enough.
Kids who are home schooled have the time and the energy to be active in ways that their public schooled counterparts can not match.