If the population is growing quickly in a small area, than the ___must be increasing as well.

Population density.

Explanation:

Population density is a statistical data that allows graphing the number of people living in a given territory, dividing the number of inhabitants of that territory by its size, usually in square kilometers or square miles.

Thus, the more the population increases in a territory, the more its population density will increase. An example of territories with a high population density is Hong Kong, with about 7,000 inhabitants per square kilometer, while an example of a nation with a low population density is Canada, with 4 inhabitants per square kilometer.