Looking at the graphs I think you've been taking them from the Curious Cat city upkeep thread? There seems to be a fair amount of debate on their accuracy even in that thread, and the graphs predate the 1.52 patch anyway. I don't think civic upkeep is purely a function of population. Take the comparison between the 9 city block and my 4 size 20 cities. The 9 city block has a population of 72, and a civic upkeep of 35gpt, the 4 large cities have a population of 80 and a civic upkeep of 28gpt. It looks like population is a major factor, but there must be something else as well judging from these numbers, so I threw it in to be on the safe side.