I remember figuring it out back in Civ2 that the "real" population of a size 1 city was 10,000; size 2 was 10,000+20,000 = 30,000; size 3 was 10,000+20,000+30,000 = 60,000; size n was (1+2+...+n)*10,000 = n*(n+1)*5,000. In that sense, a size 2 city had "more" population than two size 1 cities. Likewise, a size 15 city (1.2 million) had more population even than two size 10 cities (2*450,000=0.9 million). I haven't checked if the calculation mechanism for CiV is the same, but I'm guessing it's similar.