It would seem that they may have eaten more 'fast food' than moderns do. It is probably the kind of food one eats and the amount, along with the exercise one gets, that determines one's stature. Insofar as the army was concerned, they would not want fat men nor skinny men. They wanted strong, well proportioned men with large hands. Their breads were made from whole grains, which retain their vitamins, and not from over milled grains, i.e., 'white bread'. Their meats were probably not reconstituted, dyed, fat laden hamburgers or deli meats, such as baloney. Perhaps, there was no secret; just common sense.