There surely is a point at which upsizing becomes less efficient. But increasing farm size has been and still is, in the EU at least, government and agro-industry policy.

The combine harvester isn't the only criterion, btw - farmers aren't all pure grain producers. There's a drive in most of the EU to wipe out smaller dairy producers, for instance. It works mainly by scheduled reductions in milk prices paid to producers. That is, planned elimination of the smaller units.