Anarchy would mean that everyone would have a reasonable claim to the land

There is a natural risk to property ownership, particularly land.

If someone thinks you have taken too much land, they might want to have use of it but would be fearful that you are prepared to use force in defending your land. If so they may choose to preemptively attack you. If we seek to claim property we risk preemptive attack.

We can reduce the risk that we will be attacked by someone who wants (to have) use of our land, by owning only a small quantity. If we own a little land there is less chance of being killed, provided land ownership is not subsidised which removes (or reduces) the risk.

Owning land is risky.

Without the Government to protect them people will be naturally inclined to own and claim only a small proportion of the land available, so that they do not risk getting killed. There are no landlords in an anarchist system.