The population density of Seoul is 17000 people per square kilometer. If you take this statistic and times it by the size of seoul (605.21) it will equal the population of seoul (10288570). From These measurements you can tell that seoul is a very compact place because no matter which way you look at it 10 million people 605 square kilometres of land must be pretty compact. Last year seoul was calculated as the tenth biggest city population wise.
Here is a graph showing the population increase in seoul from 1900 to 2010. as you can see from about the 1960s the population in seoul has boomed an went from 2 million to 4 million in about a year and it kept following this trend until the 1990s when people started leaving seoul.