Just a pure speculation here...
I
think that Ruthless AI mess up AI's skill at expanded into ever more Neutral Territory.
To be clear, I'm not asking AI to expand over its economic limit
NOR into neutral territory that is not right beside that civ, OK

. What I'm talking about is that AIs in Ruthless AI
seem to prefer warfare overmuch, even when they still have neutral territory right there nearby, nothing in their way at all. Only after AIs expand as much as they can, and there is need for a conquest then should they go ahead and plan for warfare.
Remember, Afforess, I'm just speculating because I have absolutely no idea how you coded Ruthless AI. I'm posting this because this time I have my Flexible Difficulty maxed at Prince and I'm very close to Medieval Era now and I have 8 well-developing cities while the next best AI had just 3! One of civ in the list had 4 cities but it seemed that another civ captured one of them. So in this game, I have this exploitative strategy: let AIs bash each other up and stay militarily and (long-term) economically weak before I run out of nearby neutral territory, then I will conquer the entire continent I'm on

.
By the way, in the last game, it was only when AI reached mid-Renaissance that they finally increased in city numbers.
So in summary, even in Ruthless AI game option, AIs should expand into nearby neutral territory before considering warfare. Exceptions should only be for resource necessity and military might. Again, I emphasize that it is just speculation and please correct me if I'm wrong here please.