[China rarely existed in larger than its current form over thousands of years. And then not by much.]
Just how big were Chinese revolts? This big:
https://en.wikipedia.org/wiki/Taiping_Rebellion
https://en.wikipedia.org/wiki/Yellow_Turban_Rebellion
https://en.wikipedia.org/wiki/Li_Zicheng
And estimated 30m people died. Just how strong were the powers on China’s borders? This strong:
https://en.wikipedia.org/wiki/Transition_from_Ming_to_Qing
https://en.wikipedia.org/wiki/Mongol_conquest_of_China
https://books.google.com/books?id=1LUoDAAAQBAJ&pg=PT145&lpg=PT145&dq=tibetan+burn+changan&source=bl&ots=TtC_bjQlKe&sig=ACfU3U2VdT-GDx0jUEYglq3AMh-ebhw6UA&hl=en&sa=X&ved=2ahUKEwiamd_suZfnAhUE16wKHXL3AMQQ6AEwDHoECAkQAQ#v=onepage&q=tibetan%20burn%20changan&f=false
Just how unruly were Chinese courtiers? This unruly:
https://en.wikipedia.org/wiki/Sima_Yi
https://en.wikipedia.org/wiki/An_Lushan
https://en.wikipedia.org/wiki/Zhao_Gao#Coup_following_Qin_Shi_Huang%27s_death
The above doesn’t even scratch the surface. Similarly, Russia’s empire-building only stopped when it encountered serious external opposition, or it was constrained by internal resources.
I am fully aware of the Chinese history. A lot of countries has similar things in their past and don’t qualify as global empires. China was always incapable to convert its economic power into a military power and they weren’t good at diplomacy. That disqualify them as an Empire.
As for Russia her expansion was only natural. There is no need in this anymore since the concept of defensive perimeter is mostly dated.