How did people stay thin when they ate biscuit and sausage gravy, 2 eggs, milk, juice every morning?

Hi fat/cal meals. People ate huge, high-fat, high protein meals when they did heavy physical labor (farming, construction, digging ditches, etc); they needed those calories to sustain them through the day; their heavy physical activity burned off those calories; so they remained thin. Physical workers still eat > 2000 calories a day to maintain strength yet stay slim. Increased activity burns off calories.
Yes plus. You can work it off. Still you need to process the fats - makes sure you take carnitine and Niacin - the first one will turn sugars into health HDL instead of triglycerides, Niacin will process bad cholesterol into bile instead of ldl.