It's a matter of insufficient resources, not a matter of the actual population.
Except that overpopulation and insufficient resources are essentially the same thing, are they not? If there are no longer enough resources to support a population - even if that population can fit comfortably in the area that they're in - then that area has become overpopulated, and the lack of resources is because of overpopulation. It goes for us as well, when looking at the world as a whole - even though spatially we can all fit quite comfortably, our population will eventually grow beyond the world's ability to support it. The insufficient resources are because of overpopulation. Insufficient resources doesn't just happen without a cause of some sort (whether overpopulation, natural disaster, disease, or the introduction of a new invasive species).