Think of it as a bacteria... on a petris dish with the right temperature and lots of food, no competition, no enemies or predators and bacteria can grow at an astonishing rate... in fact it is mathematically logrithmic.... if it could continue at that rate within a few years there would be enough bacteria to equal the mass of the entire earth.
The reality however is that under optimum conditions this growth will occur, but the growth itself will change those conditions... when the bacteria runs out of food, or reaches the edge of the petris dish the growth can't continue.
Note I am comparing growth, not suggesting Chinese people are bacteria or some sort of disease.
They have expanded rapidly because there was enormous potential for expansion... there was lots of room to grow and increase.
The mistake people make is the assumption that the growth will continue forever, but once it reaches parity to other similar groups... ie the west or the east, then growth slows down... in other words they can catch up quickly, but wont blaze past and become superior.
If you take a child from birth from the middle ages and put them through modern schooling you would not be able to tell the difference between other children... that doesn't mean in 10 years time they will be working at the level of someone from the 23 century...