Because large farms get such a large subsidy, they can stay in business even if they're selling what they produce below the cost of production.
Subsidies rise in accordance of land area, and so reward mechanised monoculture.
Small farms have time and again been labelled as backward, unproductive and inefficient - an obstacle to be overcome in the process of economic development. The American model of large scale, mechanized, corporate agriculture is held out as the best, if not the only way to efficiently feed the world's population. Yield is how much of a single crop you can get per unit area - for e.g., bushels of soy beans per acre. That's a measure that's only relevant to monocultures. A monoculture is when a single crop is grown in a field, rather than the kind of mixtures of crops and animals that small farmers have.