The four factors that can affect population size are fertility rate, mortality rate, immigration and emigration. Fertility rate and mortality rate are often grouped together as are immigration and emigration.
The fertility rate of a population refers to the average amount of children that women have throughout their lifetime, adjusted to births per certain amount of the population in the year. The opposing factor to fertility rate is mortality rate, which is equal to the typical amount of deaths per the same amount of the population in the year. Finally, immigration is a count of how many foreigners become residents of the country, while emigration is a count of how many residents leave the country.