It comes from the days when you would have breakfast after being up for a few hours doing morning chores on the farm.    Roll out of bed at 5:00 and spend 3 hours milking cows, dragging hay and mucking stalls and you'll be ready for a big breakfast like that.  My grandparents ate enormous amounts for breakfast and lunch because they had to. One of the reasons people are so fat is because they are too stupid to realize that you don't eat like a farmhand unless you actually are one.