The commenter, who identifies him/herself as The Food Calculator, wrote,
I have read reports from from urban farm, stating that they feed 100 people. They actually sell 100 boxes of 10 pounds of vegetable per week. then they tell people, we produce tons of vegetable and people goes wow!, TONS... and people goes : if this tiny roof garden feeds 100 people, then 10,000 of them in our city, and it will feed 1 million people. we don't need farm anymore! ...
The fact is that 10 pounds of tomatoes or cucumbers or other vegetables like this, at 100 calories per pound for vegetables, is 1000 calories per week. if an adult needs in average 2500 calories per day (that's 17500 per week) then the 10 pounds of vegetable is only 6% of the calories needed.
I have run into the same problem in talking about the productivity of home gardens. By midsummer, we are all hauling in many pounds of food from our gardens each week, enough food that it seems as though we ought to all be food-self-sufficient, but we are unlikely to be feeding our families exclusively from the garden. The suburban gardeners, for the most part, are still visiting grocery stores for oil, dairy, grain products, dried beans, and more. When other people, especially non-gardeners, see the great baskets of food coming into the house, it can be hard to get this point across.
My own thinking is that, unless the gardener is growing serious quantities of staple crops that provide a lot of protein - like potatoes, sweet potatoes, beans for drying, or large amounts of corn or other grains - home gardening's biggest contribution is generally in ensuring variety in the diet and providing vitamins and minerals.
This can make a huge difference in people's daily meals, especially since homegrown veggies and fruits are so very tasty, but that isn't the same as providing all the food that a family eats. Some people, whose gardens are larger and more focused on staple crops, may actually be feeding themselves mostly from the garden, but a lot of us are growing what are maybe better described as luxury foods: our favorite tomatoes, baby zucchini, specialized greens, fancy cucumbers.
The luxury foods are highly nutritious, and I wouldn't want to be without them, but in hard times it is also good to be able to grow some of the staple foods. Each year I grow a little patch of potatoes, a little patch of sweet potatoes, and some popcorn (this year there also is some parching corn). The patches are small enough that the food they produce only lasts a few months, at most, before it is gone, but they are also enough that I have a clue about the size of garden I would need if I were to try to feed my family almost exclusively from the yard. The garden would need to be bigger!
For the five or so months of our garden's peak production, my family doesn't need to visit the produce department of the local grocery store for ingredients that help us create varied and healthful meals - the garden supplies plenty - but we go there anyway because we like bananas, mangoes, and avocados, none of which grow in our yard. We also pick up oil, dairy, grain products, dried beans, and more.