Why is size an important feature of a population?

Population size is closely linked to its rate of change. If the population is below the threshold for the resources available, it will increase in size ... The more distance between the actual size and the threshold, the greater the rate of increase. If the population is above its threshold, it will start to decrease in size. The threshold will be the equilibrium position so it will tend to wave up and down until it gets to the equilibrium. The reason real life populations don't do this is because the resources are changing all the time so the equilibrium is a moving target.