The main problem with your question is in referring to "China" as a nation instead of China being the region with which "ethnic Chinese" associate themselves. So in that regard, your premise is false: there was never some entity of "China" controlling the lands, but rather a series of dynasties headed by an Emperor and for the most part an aristocratic class/literati. It was Qin Shi HuangDi of the Qin dynasty who can most easily be credited with fostering the notion of uniting all the people in the region as a Qin people, as opposed to identifying them based on their localities. While there are large periods of time where most of the region, China, was under one dynasty and followed the standard dynastic form (Han, Sui-Tang, Ming, and though not actually Chinese, Qing) there are large periods where this is not the case. The period between Han and Sui-Tang is marked by various local Kings vying for control as an Emperor and severe warfare between these states. The Yuan dynasty was not a "Chinese dynasty" (though there has often been an ascribed, even self, sinization of both Chinghis and Kublai) instead being established and Run by the Mongols (I can link you to more info if you'd like about the Yuan structure and why it was quite different).
A lot of people have been picking holes in your question but no one's quite mentioned the big problem - China didn't "unify" in the 2nd century BC and hold onto its territory until the present, but has rather continuously expanded and contracted over the past two thousand years. This gif demonstrates this pretty well. As you can see, only the North China plain, and later on the areas south of the Yangtze are consistently governed by a Chinese dynasty - the rest of what we now think of as China was incorporated into the empire through massive campaigns of conquest, in particular under the Qing.
Also, that gif shows the territory the dynasties claimed control over - for example it shows the Han and the Tang as incorporating what is now Guangdong in the south, which is accurate in a sense but ignores the fact that both dynasties were engaged in constant wars against native tribes for control. That right there is probably the simplest answer to your question - China reached its current size through constant campaigns of war and expansion, some of which they won and some of which they lost. The Qing and Ming were relatively successful compared to the other dynasties (the Han tried to expand westwards for example, but failed repeatedly), hence China's huge size today.
As the others have mentioned, there are some problems with the premise of your question. Nonetheless I can speak a little about the Chinese aspect.
The idea that China as a single nation which has a hold onto its territory is sort of an illusion. One of the reasons is due to the concept of "Mandate of Heaven". Basically, each new dynasty, whether started by Chinese or Foreign ethnicities claims legitimacy by stating that their mandate to govern is passed on from a previous (also legitimate) dynasty. This creates a lineage of legitimate dynasties starting from mythical times, and an illusion that the Chinese empire is continuous.
Couple of problems with this. First, some dynastic houses and the ruling elites are foreign, like the Yuan by Mongols and Qing by Manchus. And each time this happens, there is some significant cultural changes (I use this term very liberally here) before and after.
Second, even in times when China is significantly fragmented, the illusion of continuity is maintained in that one faction is usually deemed legitimate even though they control only a region of what is geographically considered China. An example of this would be the Three Kingdoms where the Wei kingdom is the legitimate one, all historical references are made with respect to the era names of the Wei emperors. An even better example is the Five dynasty period after the Tang dynasty. Each of the five legitimate dynasty were merely regional warlords controlling a fraction of land, but their significance in history has been amplified by their legitimacy.
