There are things that are processed per civilization.
Diplomacy is an example. With 5 civs you have 4 AI's each processing diplomacy with up to 4 other civs, for up to 16 relationships considered. With 10 civs you have 9 AIs each processing diplomacy with up to 9 other civs, for up to 81 relationships considered. With the 50 civs you mentioned, that is 49 AIs each processing diplomacy with up to 49 other civs, for up to 2401 relationships considered.
There are also things like deciding what tech to research and what civics to use. And then each civ may have a different value for the various units and buildings it can build, as well as a different set of buildings and units that it can built. That means that each civ has to do its own calculations for those values and have a separate cache for them, which also uses more memory.
So dividing the same number of cities and units amongst a larger number of AIs will use more processing power and more memory. Not necessarily a lot more, but more.
