yep, people say it feels bigger than NYC (which of course it is), bigger than Tokyo. Reason is the city has very little suburban sprawl, everyone's housed in highrises and the whole city is literally all skyline - hence why there are very rare pics that can show the real size of the place.
Seoul, hemmed in by mountains, occupies a tiny, super dense footprint compared to other cities, and manages to cram in 26 million urbanites in the pic below
.
http://geology.com/world-cities
Like Japan, South Korea is 65-70% forested, one of the highest percentages in the world (higher than Scandinavia or Papua New Guinea). In terms of urban areas, its the densest country in the world.