I was talking about this with my friend Evgeny Tchebotarev last night:

The transfer of sovereignty over Hong Kong (also known as the handover) happened at midnight on July 1, 1997. At the time, Hong Kong had a population of about 6.5 million people and China had a population of about 1.23 billion people. But Hong Kong punched well above its weight class and its GDP as a percentage of mainland China's GDP was about 18.4% (see above). In other words, Hong Kong represented about 0.53% of the population, but almost 1/5 of China's economic output. Today, well as of 2018, this number has declined to 2.7% (again, see above). Hong Kong still possesses a number of structural benefits compared to mainland China, but its position as a global financial center is not guaranteed.

Graph: Investopedia