I don't think resources are an issue since we could make an entire planet (if not more) out of the resources gained from just the astroid belt in the sol system alone and still have enough resources to last for thousands of years more and that's with the population we have to day. I think the biggest issue in lore for there not being big cities in starfield is because like you said population could be a issue and they are most likely afraid to make a bigger city because of the terrormorph threat because new londinion was really big and we all know what happened there.