Beijing, Shanghai, Guangzhou, Shenzhen, Wuhan, Chengdu, Tianjin and several other cities are all far larger than Hong Kong.
Hong Kong had about a million people when the British owned it and it was mainly just Hong Kong Island.
The British lost it in 1945 when the Japanese kicked them out. The British tried to come back after WW2 but they could never really get their foot back in properly and the British empire pretty much collapsed.
Hong Kong has been growing bigger ever since though, there are areas known as the new territories and it has expanded about 5x larger than when the British had it.
It's part of China today, but has some autonomy under the one country, two systems idea.
It's population today is around 7.4 million.