The Territory area decreases when the Population splits up, when its demography decreases and when the territory becomes unadapted to the species.
The Territory area increases when the manpower of the Population goes up and the local lands are adapted to the species until the maximal area is reached.
Individuals of a Population move, live, feed themselves on their Territory. They defend it too and as a consequence, they may attack another Population trying to make use of the Territory. The other option is migration if the enemies are too powerful.