I believe we're moving away from basing the economy and structures in general on city size; we're moving more toward "Do you have the time and money to do it?". That's the true factor in development of anything. The number of people is helpful, but if you have the money to blow on expansion, you can just bring in people to work. This also allows lower tier cities (since it's hardcoded that barbarians can only reach 'minor city' in development) to develop effective trade networks; not every trade hub had to be massive (though most did grow somewhat large, but being 'huge' wasn't really a necessity; that's in reverse, a place was a trade hub and THEN grew, usually).