Seoul's population will likely shrink to below 10 million by 2025 but it will still remain the largest city in Korea.

According to Statistics Korea the population of Seoul will gradually decrease to 9.7 million by 2025 due mainly to the nation's dramatically low birth rate and rapidly aging society.
The agency says Pyongyang, the capital of North Korea, could become the second largest city on the Korean Peninsula in terms of population by 2015, bumping Busan to third place.
Pyongyang will likely reach a population of some 3.4 million whereas Busan's population may contract from the current 3.5 million to 3.3 million by 2015.
(Source: chosun.com)