The most plausible explanation is that small farmers use more labour per hectare than big farmers. Their workforce largely consists of members of their own families, which means that labour costs are lower than on large farms (they don’t have to spend money recruiting or supervising workers), while the quality of the work is higher. With more labour, farmers can cultivate their land more intensively: they spend more time terracing and building irrigation systems; they sow again immediately after the harvest; and they might grow several crops in the same field.

Before Monbiot even explained these reasons for increased productivity on smaller farms, I was already paraphrasing this paragraph in my head before even reading it!