There's an "Asian" climate? And even the harvest observation isn't true.
Ahhhhh. That's part of the reason. But keep in mind that China's wheat and corn harvest together has about the same tonnage weight as China's rice harvest. Basically the story is that wheat was/is the main food crop in the Red River region/Northern China. In the South wet rice is the dominate crop but that only really became a populous area after, say, the 800s when the introduction of new rice strains and massive internal migration created the conditions for the large-scale development of irrigation. The Columbian Exchange saw corn which grows well at height and on relatively poor soil and the potato which grows anywhere support further growth as new areas were opened up for cultivation. It was the latter two which supported the long boom in China's population under the Qing.
Java kind of, sorta, mirrored China. Except that its first major crop was dry rice. Oh, and population growth only really started to take off when large-scale food imports became a thing after the 1860s (or thereabouts). Prior that Java's population was large but confined to the few areas that could support wet rice agriculture. With food imports, the introduction of other sources of calories and large scale irrigation projects in newly opened up areas, Java's population boomed. So much so, that something like 20% of the population starved to death during the Second World War when the Japanese and Allies disrupted the archipelago's transportation networks.