I noticed while playing a game recently that some of my cities had gotten from level 4 to level 5 with relatively little trouble, while another one seemed to require a ton of food and housing.  When I investigated this (by clicking on the "Citizens" icon in the resources panel), I noticed that the first city listed its "base production" as being 250, while the second one listed it as being 750.  Clicking around a little more, it appears that the base production is supposed to be linked to city size; my other level 4 cities all have 750 there, but level 3 cities have 250 and lower-level cities have even smaller number.  Maybe this one city didn't get its citizen production increased when I leveled it?

Save game here:

http://dl.dropbox.com/u/9192382/Stardock/WeirdPopulationStats.EleSav