Larger cities make a big difference, I can take a look at your save out of curiosity regarding trade. I wonder if the cities of the lowered scored civs you are at peace with have lucrative trade routes to your cities, not sure if trade routes are two way streets or a one way street. These things I could check in your save, or any save really, but your save is ideal as the leading civ have zero foreign trade routes, not sure if I have such a save myself atm.
Well, I said "virtually no foreign trade routes"; there are some, but most cities have none at all, and the rest have mostly domestic routes.
By the way, disclaimer: I made some small edits to the civics XML on my computer, so when you load my save, it's going to have to take the time to replace my edits with the regular XML on your PC (at least this is what happened when I first made the edits and loaded my save that had no edits).