Overpopulation isnt focused on the land per person idea, but rather the lack of food for people and the health of the earth.
For example, it is shown that the more people there are present, the more food is neccessary. Therefore, farmers have to use industrial techniques (such as engine driven farming equipment and the spraying of pesticides) to keep up with demand. With an increased demand for food, and an increase in unorthodox farming techniques, the well-being of the earth is at stake.
**this is just one aspect of overpopulation