Advantages The availability of labour is high. This could turn to cheap labours due to the competition for jobs therefore leading to faster development due to decreased costs & higher production meaning a higher income. Higher chances of skilled workers. Business will have a very large market because of the high and wide variety of demands of goods. The population will be paying tax. More people means more money for the govt. which then means they can build hospitals and schools etc. In war situations the country with the larger population will have the upper hand.
Disadvantages Use of resources like land, oil etc. Lots of pollution. A larger country is hard to govern & control.