'Extreme Superstructures' showcases (among other things) giant airport, an underground city, the in progress tallest building, the ISS, and an indoor waterpark/tropical rain forest. Each has some claim to fame as the 'largest' something. However, it is not the size that is truly astounding - it is the unique ideas behind a few of the superstructures that draw in the viewer.
A huge airport is nothing particularly exciting, but the fact that one can - indefinitely - live one's life in Montreal without ever having to go outside gives people a particular perspective on what life could be like. You can live in a building that has access to the so-called Underground City and work within it or in other buildings with access to it with no need to ever step out during winter in the freezing temperatures. There are enough shops and restaurants within it to sustain oneself indefinitely.
The ability of a company to convert a giant building with no particular purpose into a water park complete with a long beach and a actual rain forest is also highlighted in this video. As the location of it is in Germany, it is not difficult to assume that a convenient 'beach' would be a popular escape when it gets cold in the winter. You don't even have to remain on the ground.
Though this documentary does give one a flavor of some of the giant structures built (or being build) around the world, its need to cover so many forced little time to be left for each individual structure. I found myself being intrigued by each construction in turn only to be shown the next, never fully exploring the previous. A more specific documentary on each structure would be needed to appreciate each individually, most likely.