Part of it it they ate a lot of things that our society usually doesn't now that we have the luxury to be picky. Rendered fat from any animal butchered was very valuable, instead of waste as we often consider it. Also drippings from meat, and the fat on the top of a pot of stock. Also organ meats were prized. Even animal hooves could be boiled, and large bones cracked to get the bone marrow. The lump of fat in the tail of a roast chicken was a favored treat for many people. If you find someone who grew up on a farm or ranch in the '40s or so, they still may consider meat drippings and the fatty chicken tail to be some of the best parts.

Grains and starchy root vegetables were staples, because they were a pretty good source of calories for the work needed to grow them, and they could be stored for some time. A farmer or other person of that income level would have considered two or three pounds of bread (usually a dense, wholegrain kind), a bowl or two of a lentil and vegetable soup, and perhaps one or more eggs or some cheese, to be a quite reasonable amount of daily food. Near a coastline, dried fish would have been a common protein source. And in some places and times, rather than bread, a savory porridge made of grains, and often vegetables or lentils, cooked in broth (preferably from meat or bones) was a common staple food.

As for how that much food was produced at that time, a much, much larger percentage of the population worked at producing food in one way or another.