It is not just space, this we have a lot I agree. It is also the ressources and these we have less and we are dependent on others that are also potentially ressource starved.
In last 20 years in the US and other first world countries resource consumption and CO2 emissions flattened or even reversed, whereas economic growth continued almost exponentially. We can now do way more with less resources.
Define economic growth.
It is indeed difficult/impossible to define precisely, but GDP and GDP per capita is a good start