What is the basis of the metropolitan boundaries? Does it indicate a certain population density?
Windy does not control metropolitan boundaries. One of the Developers, @dzuremar, says Windy uses a combination of mapy.cz and open street map. OpenStreetMap is completely created by users (open source), and it's free for anyone to fix, update, download and use. If you spot a bug in a map, you can fix it at OpenStreetMap.