[luthier]

A bit more details.

Cliffs of Dover does not physically have offline and online modes. Everything is online. When playing single-player, you are a server with a lone client.

The only difference with the actual multiplayer mode is that some of the planes aren't controlled by AI but by human players, so their control input flows over the network.

So:

1. You keep roughly the same amount of data in memory when flying over any size map. Larger maps do dynamically load and unload terrain data as needed as you move around but this process is smooth, especially on multi-core machines. (edit: it's smooth when you're flying your own plane; when you switch views from a plane over London to a plane over Le Havre, obviously the dynamic load process is more significant)

2. Planes do not care who they are controlled by, and the amount of resources taken up by human-controlled planes does not significantly differ from those taken up by AI-controlled planes. This is of course assuming good ping. Map size has no effect whatsoever on plane performance.