Two quantities may change in such a manner that if one quantity increases, the other quantity decreases and vice versa. For example, as the number of workers increases, time taken to finish the job decreases. Similarly, if we increase the speed, the time taken to cover a given distance decreases.
Hence when the population increases the area of land per person(the area of land a person owns) gets reduced as the amount of land is fixed. As on increasing the number of people in a room reduces the space.