I already figured out several issues of the game. And the main problem seems to me that the software do not use all the system resources in a proper way.
Usually, I play high economy-based, without unit-limits. Therefor it happens that the map gets too fast too small for all players. Not enough resources, not enough space. Moreover, the bigger maps invent a complete new way of playing. For example, you have to place your units and cities on strategic places on the map, because the time to travel through the whole mat would be too long. It is more close to a "open world".
I have several ideas for mods in combination with the bigger maps, but I didn't find the time to implement it, yet.