With 7 million people living on 1,108 square kilometers of land, Hong Kong is considered to be one of the most densely populated parts of the world. And if you find yourself in Mongkok or Causeway Bay on a Sunday (or Admiralty MTR station during rush hour), you most definitely will feel that space -- to even do such as walk about or into -- is at a premium.
Despite all this though, Hong Kong actually is far from being a concrete jungle. Look carefully around even in the more built up parts of the territory and you'll come across green spaces -- including on rooftops.
As for wilder green spaces, those who have seen my hiking pictures on this blog should realize full well that the Fragrant Harbour also has a lot of space that has been designated as country or marine parks -- with the result being that more than 70% of Hong Kong actually is countryside rather than urban space and "Asia's World City" consequently is one of the easiest places to go commune with nature that I've been in. :)