Well, I plan to be more exact with the stor details once we get down to it.
Anyways, that'd be just plain awful if people would try to make civilizations as large or even bigger than the Empire.
Large factions mean large responsibility and I wouldn't leave that to people I barely know.
Because these require considerable skill.
One of the users who criticized me was partially right that the Empire exists for being a sorts of "moderator" element.
It's a large civ which is difficult to ignore albeit their internal affairs and slow FTL prevents them to do large military actions that threaten a the galaxy.

That being said this doesn't mean any other civilizations need to be tiny.
I just don't expect them to be giants.
Which is actually a boon since for example a civilization with 10 times less territory as the Empire can concentrate its forces roughly 10 times better at average.
Size is a liability after a certain point.