It was more of a storage problem than a production problem. Plenty of grains and foods people grew throughout known history were more than capable of providing all the calories someone would want or need without crazy effort. The problem is keeping that food good for an entire year, or more if there is unexpected bad growing weather. One person, even using entirely hand tools and no draft animals, could grow 10x more food than they need in a year. But people didn't do that because they knew they had no way of keeping food around for that long, all it would do is attract pests and/or mold and rot after enough time.