An Aimag is kind of all over the place though, as far as composition goes. That has some tactical advantages, as your opponent also can't really prepare, or think they're "done" when they've accounted for what they consider an Aimag to be.
I also thought that either FM or ER3145 equates them as a small Galaxy, but I could be misremembering.
Generally speaking, I'd consider them big enough to deter piracy, but small enough to avoid major concerns about whichever nation they're in. These days, I'd want more than a Cluster to achieve that, though a full-blown galaxy seems overkill, so maybe it's less memory, and more presumption that provokes the prior thought.