Trump mentioned that the USA may become larger however, to do that, all he has to do is reclaim the millions of acre that have been sold off to other countries over the years.

Approximately 15.6 million acres (around 3% of private agricultural land) are owned by foreign entities, with: Canadian investors holding around 29% of this land (approximately 4.5 million acres) The remaining 38% (approximately 5.9 million acres) is held by entities from almost a hundred other countries. Another mentions that 15.6 million acres are split among around 100 countries, including India, Saudi Arabia, and Iran.

Just reclaiming our own soil will do that.