Migration affects population size by increasing it in areas where resources are plentiful and decreasing it in areas that are struggling for resources. Migration affects specific areas, but does not have an impact on the overall population size of the planet.
There are multiple reasons for migration, with the most important of them being the search for better resources and to flea conflict. Large-scale migration can result in an imbalance between sustainable resources and consumption. For this reason, many countries limit immigration. Balanced migration is the solution proposed by many countries as a compromise between the needs of a society and the appeal for migration.