Modern industrial agriculture actually produces far less per acre than small market gardening by hand does. The efficiency of the industrialized approach isn’t in output per acre it’s in output per person hour.
So if large numbers of people farmed by hand we’d actually be able to feed more people, but the trade off is far lower output of other industrialized goods and services.
It’s really not hard to understand why if you think about it just a bit. Industrial farming requires very specific growing conditions (e.g., single crop per row/field and in a spacing that allows mechanized seeding and harvest). Growing by hand allows optimizing the ground space by, for example, under planting one crop to another (e.g., radishes can grow quickly so can be sown between larger plants while those larger are still small and the radishes are harvested before the space is needed by the larger plants). This is all very labor intensive, of course.
Another example is starting virtually all seedlings inside and then transplanting, which virtually all market gardens do. That gives a head start to your plantings. But of course again it’s labor intensive.
