I never said "average" I said "many people". A 1 lb loaf of bread is only 1/2 the calories needed for an ordinary man. Doing hard work in weather under 57 degrees F, a man needs 4,500 calories, according to the U.S. military.
4-6 slices at breakfast is easy.
Another 4 slices at lunch, and 4 more with dinner for a farm family.
But, of course, this all depends on what else there is to eat. With lots of other food, less loaf bread is eaten. Grain in other forms is often substituted, rolls, doughnuts, bagels, cereal, crackers, etc.
But the discussion was about how much land was required to feed a family. Cereal crops are some of the most efficient for providing the calories necessary. In Medieval Europe, grain was converted to ale, later to beer. It provided lots of calories as well.
Other crops require more land. Growing crops in a subsistence manner is hard work, so it requires more calories.