Make cities cost resources too, scaled similarly to costs. City 2 could be (whatever cash it is), 60 steel, 50 alum and 20 gas. City 20 could be 15k steel, 10k alum and 5k gas ect. Places a harder soft cap on cities, and means people will be using a significant amount of resources when at a higher level, whereas top and super tier nations as present have a massive + net production; as more nations are reaching the upper tiers, the more abundant resources are getting. Answer: make upper tier use more