I worked part-time at a McDonald's for nearly a year in 1971. Most adults ordered what is now the equivalent of a child's Happy Meal. A regular hamburger (about 3.7 oz, 250 cal, ) or cheeseburger (300 cal), regular-sized order of fries (2.4 oz, 200 cal) and a small drink (12 oz, 140 cal) - no refills. So approx 590 calories. Mind you, this was an adult meal, not a snack.
Growing teenaged boys and young men ordered Big Macs. Large fries (the kind not in the bag but in the red box) were sold far less often than the little bag of fries and were often split between two or more people. Those are now sold as "medium" fries.
These days people typically order Quarter Pounders w/cheese (7.3 oz, 520 cal) or Quarter Pounder w/Cheese Deluxe (9.2 oz, 630 cal)), Large Fries (5.9 oz, 490 cal) and medium or large sodas (21 or 30 oz, 240 or more cal) plus free refills. Approx 1250 calories.
Going into a McD's or other fast food places at about 3:00 pm - when schools have just gotten out - is a quick lesson on why the obesity crisis exists. Droves of adults, parents, children and teens, buying and wolfing down full meals of burgers, fries and sodas as between meal snacks.
When I worked at McD's, business between mealtimes was almost non-existent. We didn't even open until around 11:00 for early lunches - breakfasts and Egg McMuffins were still in the future - and we closed by about 8 pm.
That's just one small snapshot of why the obesity crisis has become so severe over the past 50 years.