Answer:
This is a rather simple question. Let's put it this way. If you add more marbles to a container, there are going to be more marbles in the same container. On the other hand, if you take away the marbles from the container, there are going to be fewer marbles in the same container. The same thing could be said with people. If more people migrate to an area, the area becomes denser since there are more people in one place. However, if people migrate away from the area, the area is going to be less dense since there are fewer people in one place.
That was a strange analogy to make, but I hope you understand it.