Seoul is a very, very big place.

The standard city contains about twelve million people. That is a lot of people. A lot. However, all of these people are not crammed into a space big enough to really accommodate them all. Hence the almost dystopian abundance of high rise buildings into which hundreds, if not thousands of bodies can be squeezed. Now London has an official population of about seven and a half million. However, these people are put in a space about three times the size of Seoul. It is this kind of comparison that elucidates the fact that Seoul has the densest population in the developed world. Only the developing world beats them out, India with its dysentery and roaming cows and Cairo with its... honestly I do not know what is in Cairo. Anti americanism? Buses to the pyramids?

The wider metropolitan area of Seoul just gets even more massiver in its largeousity. Greater Seoul, including the satellite towns and Incheon (where the airport is), has about twenty two million people. In terms of official statistics, this makes second only to Tokyo.
Now Seoul is split up into 25 districts known as Gu. I live in Nowon Gu, the most densely populated Gu in all of Seoul. It has roughly the population of Glasgow, about six and a half thousand crammed inside an area roughly the size of the Beer Bar. Actually the area is about 35.44 km² but honestly folks that is not that big. Glasgow itself is in fact has an area of 175km. So the same amount of folk in a place a fifth the size. If Keanu Reeves was reading this, that would surely warrant a 'whoa.'