This is because Asia has certain fertile regions that allow multiple crop cultivations or large crop harvests. The three most prominent regions are central plains in China, the Gangetic/Brahmaputra plains in India, and the island of Java (whose adequate rainfall can allow up to three rice cultivations a year). In addition, there are other fertile regions as well.

India and China have been subject to multiple invasions for this very reason.

hi! there's lots of room for more input, but you'll find some info in this section of the FAQ*

Why do China and India have such large populations?

*see the link on the sidebar or the wiki tab

Fertile farmland, long growing seasons, good rainfall.

You will note on this list of historic famines that China and India feature prominently.

http://en.wikipedia.org/wiki/List_of_famines

The greater population densities allowed by their superior food productivity also left those areas more vulnerable to mass starvation if crops failed before the era of international bulk-cargo transport.

I can't speak to India or Southeast Asia, but with the dominance of Confucianism in the Han Dynasty, farmers were recognized as vital and enjoyed social status near that of nobles and state bureaucrat (in theory, at least, in times of social breakdown things got iffy); along those lines, massive state-sponsored geo-engineering projects were undertaken to optimize farmland. So instead of the pattern in western Eurasia of warrior kings ruling serfs who had little to no capital, little direction in the cultivation of their land, and no geographic coordination to tame the, eg, river floods or other adverse events. Look at the Little Ice Age towards the end of the High Medieval era; climate shifts triggered generations of wars to secure farmland instead of supporting the native farmers and engineering solutions to ensure food production.

With Southeast Asia, they have had three things going for them: 1) genetic epicenter: home range of all citrus, the original chicken, all the spices you could want, etc. 2) trade intermediary between China, India, Persia, etc. 3) a constant flood of immigration as the expanding Han pushed Tibeto-Burman tribes out of Yunnan and into the SEA peninsula. You can conflate #2 and #3 into thalassocracies like the Champa peoples in what is now south/central Vietnam, who then, being pushed out by consolidating mainland empires, contributed to the Polynesian expansion.

I don't know what to say about India. As near as I can tell, since the Toba eruption in 60k BP, India has been the last stop for every kind of crazy mobile tribe that ever headed that way. It's like the Baskin-Robbins 31-flavors of linguistics.

tl;dr: Asians really love to eat and will spend capital on butter rather than guns.