How did China become so populous and why are its wars historically so deadly?

Basically, Chinese agriculture was extremely productive, and the state took measures to make transport of food across provinces cheap and safe; three hundred miles over water is much shorter than twenty miles over rough terrain, and the network of canals linking important towns along the Yangtze with each other and the north greatly facilitated grain shipments; a shipwrecked Korean sailor during the Ming recorded making 85 km by the canals in a single day, and when you can ship food overwater, less is going to be eaten by draft animals along the way. The wars are so deadly in part because of simply having such a massive population; even if it was the same percentage of population as a European war, it'd still be terribly bloody. However, what's peculiar to Chinese history is that it's relatively rare for them to engage in the kind of relatively equal, state on state warfare you see in modern Europe; it's usually a rebellion from within the country, which directly and immediately damages the agricultural infrastructure of the country, leading to famine and disease much more quickly than simple army vs army conflict on the border between two roughly equal states would, or, in the case of the Yuan and Qing, wars of conquest that last for decades.