Hong Kong is generally quite mountainous, and the most built up areas on Hong Kong Island and Kowloon, while impressive, only cover the core of the territory.
When Hong Kong fully returns to China in 2047, maybe it'll be counted as part of Shenzen's metropolitan area. The 13th Five Year Plan mentions a long term vision for linking all the cities in the Pearl River Delta into a megalopolis. >>529508
The Tokyo metropolitan area is nearly three times the size in terms of population.
Although LA is massive in terms of sprawling physical size.