I think market forces will control growth for the sake of growth.
The sustainability debate and population debate are the same thing though. The expansion/pollution/waste is the problem, right? If you control that then numbers become less and less relevant. The problem is the damage we're doing to our environment and that is caused by our behaviour. A city of a million that uses only coal, oil, doesn't recycle, dumps waste into rivers, etc. etc. will have more of an impact than a city of 5 million that uses wind/tide/solar/whatever, recycles at low cost, treats any waste, etc. etc.
Numbers are the wrong argument for so many reasons. I'll take 100 butterflies in my loungeroom over one wasp because their behaviour and consequences are totally different.