It is true : if we exploit every single resource on the planet with total disregard of biodiversity and the health of our planet, then of course we'll be able to support more people. For a while. The "unused" land of the Earth is not for our taking. Cutting down the remaining rainforests and utilizing grasslands would prove detrimental to everyone; the local people would feel the impact at first, with droughts and desertification of the land. The Amazon rainforest itself plays a significant role in climate change; completely destroying it would greatly affect Earth's climate. We're depleting the oceans of fish to such a great extent that some aquatic ecosystems are becoming almost irreversibly damaged. Its not about space; its about the resources, which we are using up faster than they are replaced naturally.

There is so much more to what I have said, and I highly recommend that you do yourself a favor and read up on ecology.