The MT in that usage is metric tons.
1 megaton is 1,000,000 tons.
1,500 megatons would be 1,500,000,000... 1.5 BILLION tons...
USDA estimates total worldwide rice production for this growing year at 503 million metric tons.
Virtually everything that the US exports to Japan is Japonic rice, which is the primary food rice is most of Asia.