As others have said, it's not the space each person needs, it's the resources each person consumes and what do with the spent ones. Think about how much trash you've generated in your lifetime. It doesn't just disappear, it has to go somewhere. What about all the sewage you produce? Think about all the food you eat - how much land and fresh water is required to sustain that? And how many chemicals are we dumping onto the land that ends up in our water to get the massive amounts of food we need? And what about the materials for your house, cars, clothes etc? Keep in mind several billion people currently live in extreme poverty and don't have cars, don't live in homes with electricity and running water, and don't regularly eat meat. When those people move up in income level, how much more forest will we destroy to make farmland for them? How much more trash will they produce etc
So, it's not a question of living space per person. It's a question of total resources required to support billions of people living modern lifestyles.