Population growth in a country using a capitaist economic system means more demand for products and therefore economic growth. For a country with a low population and a lot of land, it can be an advantage especially if the distribution of wealth remains close to level so everybody can prosper. That's not a long term solution for the planet however.
The disadvantage is that population growth puts tremendous pressure on economic and ecological resources. In the long term, the world will have to come to terms with the fact that we are consuming at a faster rate than nature is able to produce right now. The consequences of not making an adjustment in an orderly manner will not be a pretty sight.