While with the same land mass as China, we have more than a dozen countries in Europe. And in comparison to Japan, the UK has several kingdoms. (with Russia, I think it's because most of its eastern parts were sparsely populated and with India, I don't know, maybe because of the brits?)
There were the Ryukyu Kingdom and the Ainu people in Japan. And China had several kingdoms. Why then, are they so homogenous now (compare to Europe)?
Check out this gif - it shows how the concept of "China" has changed over time, different regions, dynasties, kingdoms, and so on. As you'd imagine, the entirety of modern China is probably not composed of one single "ethnicity" of people (as others have pointed out) - instead, you have more than fifty different ethnic groups with Han Chinese being the vast majority. The idea of this ethnic group sprung largely from the Han dynasty, where a large region of "China" was established. Surrounding minor ethnic groups were, well, minor ethnic groups - these were assimilated into the Chinese "fold" more or less. The government would send specific officials to these groups to educate them and, for lack of a better word (it's late), indoctrinate these minor ethnic groups - this meant language and culture, among other factors and customs, which leads to the creation of one identity. Since the unifying Qin dynasty, the ruling governments have used this method to relatively peacefully build and consolidate their power over a large area - this naturally builds a strong sense of solidarity and a "Chinese" ethnicity, which is true in many cases. However, even today there are still many different dialects, customs, and so on being practiced in various regions of China and internationally (e.g. South, North, Taiwan people, Cantonese and Mandarin languages) - so the question why China feels more homogeneous can essentially be answered with "because of our perceptions of their identity". Your average Chinese person will easily be able to point out a whole slew of differences between their ethnic group and another, or even between their region and another (e.g. Beijing vs Shanghai vs Fujian etc.), however to the West they are, whether due to a lack of information or simply a Eurocentric perspective, one homogeneous group.
Follow-up: Was there a significant differences in Eastern and Western feudalism? Did that have anything to do with the stratification of Europe?
OP, it would be worth x-posting this question (on China) to this AMA that recently opened
Related question: I have heard people speculate that the Imperial Exam system used in China effectively brought people under the Han/Confuncianist cultural sphere, since anyone who wanted a nice job as a bureaucrat had to take the exams, and taking the exams required a standardized set of skills/knowledge/languages. Is this accurate at all, or is it overstating the cultural impact of the exams?
