It's made up of about 500 islands, of which 110 are populated. Copenhagen is not the biggest city in the world, land area wise, and it is surprising it can have that many residents. They accomplish this by having many, many apartments, and cramming everything into small spaces. Just about everything in Denmark is smaller than the same thing in the U.S. Houses are smaller. Yards are smaller.
Cars are smaller.
Streets are narrower. Space is not wasted in Denmark - probably because there isn't a lot of it. But nothing else is wasted either. They lead the world in many things, including recycling. Aluminum cans are non-existent, and all bottles are recycled.