It's worth remembering that China is a very big country, and considerably bigger than Alexander the Great's empire ever was. Wikipedia says that Alexander the Great's empire was roughly 5.2 million square kilometres in area, which would make it the 7th-largest country in the world just behind Australia. The Han empire on the other hand managed to have an area of around 6 million square kilometres even by the first century BC, covering most of Eastern China as well as Northern Vietnam and Korea. The Qing Empire was actually bigger than present-day China, and included all of Mongolia as well as Eastern Kazakhstan and the Russian Far East. So China actually conquered a considerably larger area of land than Greece ever did, not much longer after the fall of Alexander the Great.