Anarchy has great potential of helping environment by decreasing number of people.

It does not follow that the population would decrease due to a mere absence of a state.

Population will not decrease directly from absence of government, but:

1. Many more people will be killed by criminals and in resulting civil wars without government provided security.
2. People will die from hunger - bacause modern society is not hunter / gatherers, they depend on investments (like planting seeds) for their food and other supplyes, and investments will not exists without government provided protection of private (or any other) property.
3. If there would alternative providers of protection - like private / clan armies, mafia, ... - they will naturally fight for dominance.
4. After some competing clans will divide country in zones of influence, which will result in feudal like society which is economically inefficient (with big barriers for trade) and often as far from freedom as it gets - with unlimited power of local chiefs.
5. If you dion't believe me - there are many examples of countries without acting government - Afganistan some time ago, Somalia now, Lebanon some time ago before Syria occupation, Chechnia some time ago before Russian occupation,Congo now.