Over population is the problem that underlies most of the major world problems. Population is the direct cause of all resource demands. Water, food, shelter, energy, infrastructure, space, transportation, health care, etc. are each directly related to population. Each resource requires both extraction and waste disposal with the inherent ecosystem disruption and damage, and limited supply. However, these are not the most important problems.
This is not the Malthusian "population is limited by subsistence" argument nor a "save the planet" environmental argument. The problem is not limited resources or damage, because technology and strict control can sustain a nearly unlimited population without undue damage to the environment.
The problem is the quality of life. Do we want to live squashed into urban containers with trips outside limited to a once-in-a-lifetime pilgrimage? Freedom of action and freedom from coercion are the first casualties of congestion. Also the institutions of limitation are a magnet for the domination and control personalities, so oppression is unavoidable.
The only realistic solution is to force people to live within their own resources. This means severely limited migration. People migrate from the limited and controlled to the freer. Unfortunately this leads to a new congestion which in turn leads to new limitation and control. The only enforceable boundary is currently the sovereign state. Each state must bear the consequences of their own population choices. There is no ethical problem here, no one has the right to migrate to another's living space, no matter their problems at home.