Portion sizes were smaller across the board. Soft drinks were modest. Fast food existed, but it was not oversized. A burger was just a burger, not a stacked tower of bread and meat. There were no automatic upgrades or oversized combinations. Food was meant to satisfy hunger, not overwhelm it. Over time, this prevented the stomach from stretching and reduced the urge to overeat.
Television also played a limited role. It had schedules. You watched a programme and then turned it off. Children were expected to go outside after a short time. Meals were eaten at the table without screens. Eating and entertainment were separate activities. Compare that to today, where screens follow people everywhere and meals are often consumed while distracted. In the 1970s, daily screen time was minimal. Today, it can exceed half the day.