What are the main economic problems due to overpopulation?

While the first answer is somewhat correct, a growing
population does not necessarily mean that demand will grow faster than
supply.

In some ways, there is no such thing as
overpopulation.  There can only be too much population compared to
economic capacity.

If there are too many
people compared to a country's economic capacity, the country will experience low GDP
per capita.  It will probably have poor human resources (lack of education, for example)
and will have low productivity.