1. Beijing's smog crisis is not mostly due to smog produced with the city itself, therefore even if all the smog produced in Beijing was reduced to zero, Beijing would still have a bad smog problem thanks to Shanxi, Hebei, Henan and other nearby province's coal mining, coal-fired power plants and other heavy industry activity such as concrete and steel production sites that has very poor environmental regulation and oversight.
2. Beijing's population is already basically 22 million (technically 21, 240, 000) according to the United Nations. Go to page 14; http://www.un.org/en/development/desa/population/publications/pdf/urbanization/the_worlds_cities_in_2016_data_booklet.pdf
3. When you consider both of my previous points this entire story is very frustrating because these promises, even if fully delivered, will not solve the smog issue. Additionally I don't see what the issue with Beijing's population is. What are they blaming it for and why are officials aiming to "cap it"? If it's about smog then look at the Tokyo area, it has way more people and very little smog.
Money spent = money bribed