In monocultures, you have rows of one crop with bare dirt between them. In ecological terms, that bare dirt is empty niche space. It's going to be invaded and taken advantage of by some species in the ecosystem, and generally we call those species weeds. So if that bare dirt is invaded, the farmer has to invest labor or spray herbicides or pull a tractor through to deal with those weeds. Large farmers generally have monocultures because they are easier to fully mechanize.
Smaller farmers tend to have crop mixtures.
What that means is that the smaller farm with the more complex farming system gets more total production per unit area, because they're using more of the available niche space. It might look like the large farm is more productive because you're getting more, say, soybeans per acre. But you're not getting the other five, six, ten or twelve products that the smaller farmer is getting. And when you add all of those together, they come to a much greater total agricultural output per unit area than the larger farms are getting. There are a lot of reasons why smaller farms produce more per unit area than larger farmers.