AI not expanding

I am playing with a custom race (close to Terran Alliance, but I start with slightly different technologies, among which stellar cartography) on gigantic maps (always) and between rare and occasional habitable planets (I am still trying to determine the right mix, suitable for my enjoyment).

After having worked my way up to "tough" difficulty level in my first couple of games, and finding them a tad too easy, I decided to go for the "painful" difficulty level, where all AIs are "gifted". This level of difficulty is not described in the manual, but lies between tough/intelligent and crippling/genius. I am starting in the lower right hand corner of the map.

I start researching straight to trade, building the diplomatic translators on my way, and then onwards towards interstellar republic, which I am maybe one third through.

I send my flagship on anomaly hunt, rush-build a scout, and then chain build colony ships. I send my scout and colony ships to explore towards the center of the map, where I can see a series or fairly large star clusters: "there's gotta be life and planets ripe for colonizing out there". I decide to leave the few lower-class planets (class 4 through 8ish) I find on the way, which are closer to home, for later. I build a couple more scouts along the way, throw in three freighters when I discover trade (and send them to far away, yet unexplored clusters), and then resume chain producing colony ships. I eventually start colonizing a few class 10-12 level planets when I find them close to my borders (to further expand my range). I set these to chain produce constructors as soon as possible, and grab maybe 5 or 6 nearby special resources. In other words, I'm off to a fairly classic, solid start, without rushing things (I haven't rush built a single ship, except my first scout).

I am now 81 weeks into the game, and I have 12 colonies. I have met four alien civilizations: Iconians, Altarians, Yor, and Drengir. With my excellent diplomatic skills, I have managed to gain 5 or 6 technologies through trade, in exchange of xeno research and some money. My treasury, by the way, is doing great, thanks to a series of lucky credit-yielding anomaly explorations, and this, even though my spending is at 100% and I lose 100ish credits per turn. At the current state of things, I should be able to keep up this level of spending for another 60 turns. So things are pretty going well, even though I don't feel like I have been outrageously lucky overall.

But here's the thing. My opponents don't seem to be expanding. I am playing with blind exploration, so I can't see their whole territories, but from the diplomacy screen it seems like they each have their home planet, and only one additional planet (that level 4 planet usually present in home systems). What more is, in my latest turn, I just grabbed four (inluded in the 12 mentioned above) very juicy planets practically in front of the Drengir's nose: respectively class 12, 17, 18 and 26. Three of those were colonized by ships that had to travel for probably 30 turns or so all the way from my home system.

Has anyone else had this experience of the AI not expanding beyond their home system? I am at a point where I feel this game will be too easy. I would like to know if this is normal behavior for the AI, or if there maybe is a problem with the "painful" difficulty level? Any feedback is appreciated.

Thanks.