I generally agree on all points above, but mostly with:
Yep. I think the devs validated their software on small maps, and then did scalability testing on the larger maps (to test memory, performance, etc.). The AI for large maps was probably not validated extensively. Takes a long time.
The biggest reasons being:
1) Ship speed. Face it, the AI still doesn't build "fast" ships. Not even avg speed in my book. This makes the AI terribly slow on larger galaxies.
2) trade doesn't work worth beans on larger galaxies with lots of planets, and yet the AI uses a lot of resources there.
3) The colony rush can go on for a long time on a larger galaxy. This often seems to drag the AIs down.
4) Anomolies. A human will build more survey ships and keep thier economy going, where as the AIs rely on a single survey ship.
5) The AIs may not switch into resource grab mode fast enough, allowing you to gobble up the key resources on a larger map. (while they fight over class 3 planets)
However, I don't like the smaller maps. My favorite part of the game is the colony rush, and the transition to mid game. That just ends way too fast on a smaller map. Large is about as small as I'll play.