The answer is that science and technology have completely transformed agriculture in the last two centuries.  In fact, it was just about the time that Malthus was putting his thoughts on paper that science began to enter agriculture systematically with Joseph’s Priestley’s discoveries half a century earlier that led to an understanding of how plants turn chemicals into food powered only sunlight.  Priestly was able to keep a mouse alive in a sealed jar with a plant, which he concluded gave off  with what he called dephogistinated air, later called oxygen.  Over the next two centuries, science and engineering powered enormous gains in agricultural productivity.