Why are urban populations getting bigger?

An increase in a population in a city can be because of two main reasons. The first is natural increase. This means there are more births than deaths in the city and the population increases naturally. There can also be more deaths than births, this is a natural decrease. The second reason is migration, when people move to, or from, a city. These days it is very common for people to move from the countryside to cities. This is called 'rural to urban migration' and it increases the population size in cities.
People move to cities in this way because of push and pull factors. Push factors are reasons to leave their rural home, for example high unemployment and a lack of facilities (transport). Pull factors are reasons to move to the city, for example better education and higher wages. There are many different mixes of push and pull factors that cause people to migrate from rural to urban environments and increase the size of urban populations.