I'm not sure how much additional cost savings you get once the farms are about a square mile in size. The limiting factor on mechanization is the amount of ground that a combine harvester can cover during the acceptable harvesting window. Beyond that, you need additional equipment and people at the same rate.

My sources say that beyond that point, what is being done is shortcuts that result in less efficiency. Unskilled workers don't optimize the irrigation system and treat the capital equipment poorly, so the savings in lower wages are cancelled out by less production...