I've drilled down into this and unfortunately my conclusion is that there is not much we can do that has no other downside 
The key scaling factor (as you'd expect) is map area. Unfortunately the game engine just isn't very scalable in this regard, and since we don't have access to its code there is not much we can do to address this.
In detail it turns out that after loading the game state the bulk of the memory allocation (and load time actually) occurs while the game engine scans the game state and sets up plot graphics. It builds (and appears to retain in memory always) tile graphics for every tile, which involve a number of layers if the tile has any features, improvements, routes, yeild values (the plot yield graphics are kept for every tile for example even if you have their display turned off). The game engine also allocates an updater object (no way to tell how large this is - all we ever see is a virtual function table) UNIQUELY for every tile that has any feature or improvement on it, and binds that instance to that plot (no sharing of the updater objects between plots of an equivalence class for instance).
The upshot of all this is substantial memory usage that is proportional to 'not entirely plane' plot count (which in turn is roughly proportional to map area, but increases somewhat as games progresses and routes/improvements and so on get laid down).
There are only two things that can be done, given the code and assets we have control over:
1) Reduce map size - that's in the hands of the player, but C2C really benefits from large maps so it rather sucks as an option
2) Reduce plot graphical detail. I believe a less detailed (i.e. - smaller texture graphic) tile set would help significantly. It really requires some experimentation to see how much this might help. Since I'm not skilled in the use of any graphical editing tools it's beyond me, but what I will do is just substitute a base BtS-simple plot texture (grassland say) for ALL the terrain types and see what the effect is on memory usage.
If the experiment on simplifying terrain graphics works out then we'll need someone who IS skilled in grahpics editing (Hydro?? SO??) to produce a less detailed (well smaller in file size/memory usage anyway, however it is achieved) terrain graphics set. I suspect they can be reduced a fair bit without major compromise to the visual impact.
Edit - ok I'm stuck. Where do I get tools that can deal with the current BtS format for PAK files? Any tutorials on how to pack and unpack things you can point me at (that are up to date, but not so up to date that they don't apply to BtS but only to Civ5)?
The key scaling factor (as you'd expect) is map area. Unfortunately the game engine just isn't very scalable in this regard, and since we don't have access to its code there is not much we can do to address this.
In detail it turns out that after loading the game state the bulk of the memory allocation (and load time actually) occurs while the game engine scans the game state and sets up plot graphics. It builds (and appears to retain in memory always) tile graphics for every tile, which involve a number of layers if the tile has any features, improvements, routes, yeild values (the plot yield graphics are kept for every tile for example even if you have their display turned off). The game engine also allocates an updater object (no way to tell how large this is - all we ever see is a virtual function table) UNIQUELY for every tile that has any feature or improvement on it, and binds that instance to that plot (no sharing of the updater objects between plots of an equivalence class for instance).
The upshot of all this is substantial memory usage that is proportional to 'not entirely plane' plot count (which in turn is roughly proportional to map area, but increases somewhat as games progresses and routes/improvements and so on get laid down).
There are only two things that can be done, given the code and assets we have control over:
1) Reduce map size - that's in the hands of the player, but C2C really benefits from large maps so it rather sucks as an option
2) Reduce plot graphical detail. I believe a less detailed (i.e. - smaller texture graphic) tile set would help significantly. It really requires some experimentation to see how much this might help. Since I'm not skilled in the use of any graphical editing tools it's beyond me, but what I will do is just substitute a base BtS-simple plot texture (grassland say) for ALL the terrain types and see what the effect is on memory usage.
If the experiment on simplifying terrain graphics works out then we'll need someone who IS skilled in grahpics editing (Hydro?? SO??) to produce a less detailed (well smaller in file size/memory usage anyway, however it is achieved) terrain graphics set. I suspect they can be reduced a fair bit without major compromise to the visual impact.
Edit - ok I'm stuck. Where do I get tools that can deal with the current BtS format for PAK files? Any tutorials on how to pack and unpack things you can point me at (that are up to date, but not so up to date that they don't apply to BtS but only to Civ5)?
