Second, imagine you would like to deploy that vast military capacity in an activity other than defending China against attack. Perhaps you'd like to enlarge the Chinese sphere of influence in the Senkaku Is., on Taiwan or elsewhere in the region.
Third, you realize that any government remains in power only if it's people, at some level, acquiesce in what it does. You also realize that your enormous army is staffed almost exclusively with men who are what we call "only children," because the official one child policy was in effect in China for decades until recently.
I speculate China's leaders militarily feel their hands are tied by the unintended consequences of the one child policy, as far as adventures beyond China's borders are concerned. This explains why the government has put so much effort into convincing people that various islands in the China Sea are historically part of China, so Chinese parents can rationalize the nation spending irreplaceable young lives in their capture.