Why do they eat more rice?

There are more Chinese men/people than Japanese. It stands to reason that the total of what a greater population eats is greater than that of a smaller population.

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China is at present suffering from a problem of overpopulation.

Hard to say, but my guess would be cultural differences. China is one of the largest countries so they have a vast amount of land to produce rice in lesser expensive prices. So more Chinese people would eat rice over Japanese people.