Unless OP are willingly to have ASB or something create many Volcanoes or large rivers like Yangtze to fertiling its land
Sumatra seems to have about as many volcanoes as Java.
Unless OP are willingly to have ASB or something create many Volcanoes or large rivers like Yangtze to fertiling its land
Could the soil improvement methods from the Amazon Basin have made an impact on the other islands?
Any take on why Javanese/Balinese agriculture didn't make it to Papua New Guinea ? Not having visited java but have seen rice paddies in the mountainous Ubud ?
Fertility of soil is depdent on technology absent of natural causes. Also,You can have set of bunch of small rivers which are not noticeable by map nor by people in the forums which can function like the Yangtze as long the people knows how to harness those. You also got deep wells, etc.
That is why population can be brought, maintained or even expand in south east Asia regardless of number of volcanoes. Otherwise, if fertility is dependent on volcanoes even the Yangtze won't be able to maintain that kind of population.
That's why population has always been centered on where the politics lie. Otherwise you won't have mega cities like rome or Constantinople which doesn't have a huge river the size of Yangtze river nor large a large volcano within its area.