I can't argue that, because I don't have more space, so I can't do the experiment. My veggie garden is that bed and four others, plus a bunch of flower beds that veggies and herbs are starting to sneak in to
. But it is a little hard for me to imagine. Last year, the one 10 foot row of spinach gave all the spinach two of us could eat (eating spinach in salads, omelets etc several times a week) plus some to freeze for several months. Two tomato plants gave hundreds of full sized tomatoes. The beans did get kind of crowded out and did not do real well, but the pepper plants did great. I had enough garlic to eat for months and still have some for seed - this year my garlic was started from my own home grown garlic for the first time. The squash I put in there last year did not make it, but as noted I always have trouble with the vine borers, I don't think that's a function of crowding. But leaving out the squash and beans, everything I had in that bed last year was healthy and thriving and very productive.
Too early to say about this year, except the spinach doesn't seem like it will last as long. Cold spring meant it was slower to get going and then quick warm up, it is trying to bolt already.
This bed is only raised about 8", but it is a spot where a tree used to be. I burned the tree and roots out, which left a lot of hole to fill, so it is soft loose soil a long way down. It is my most productive bed, because the ones in the backyard are too shady, don't get enough sun. So they do ok, but not as great.
My lot is 1/3 acre, but the back half of it is a steep wooded hillside, useless for growing much food (though I am working on putting in some nut trees, etc). The remainder has house, deck, patio, and lots of flowers, a bit of lawn, a gigantic old lilac tree. And yet I grow a whole bunch of food, including a ton of herbs in containers on the deck, and outside the veggie beds, raspberries, asparagus, strawberries, rhubarb. Two of my veggie beds sit on top of the concrete patio to make use of that space.
I am working on using as much as possible everything my "land" produces, so I am making jelly from the lilac blossoms (and last year I made chrysanthemum flower jelly), using the black walnuts from the big old tree, eating the edible weeds, etc. The more I do that the more amazed I am at the bounty of my little city lot.
So I got a little off the track, but I think you get my point.