If we concentrated the population, we would not grow as fast. But that all depends on concentrating the population and then not allowing it to expand beyond some set borders, which doesn't seem like it'd go over well. Just look at the little countries running out of land, looking to their neighboor's turf as prospective expansion.

You'd need government or fences or armed forces preventing people from expanding- it is natural for people to expand.

Maybe I am incorrect in thinking so, but my idea was that while the birth rate may be lower, we'd (in the end) approach a higher net population density if we slowly sprawled from a single super-dense hub.