Well, firstly, I wouldn't characterize a station of 40 million plus as a Big Dumb Object at all. That's a bit of an unnecessarily pejorative term I think.
In terms of important sites. It's not just that it takes people to run them (though that does matter), it's also that important sites attract people, and then people attract other people because of the economic activity. High powered corporate CEOs need services and that's provided by people who need services of their own etc. The ripple effect of all of the important sites in Absalom, to me, needs to be more significant than a middling sized Earth City.
As for ease of travel. Tt's not just interstellar travel that the Station has an advantage in. It's interstellar *trade* also as defined by the fluff. All Drift roads lead to Absalom Station pretty much literally and if people are going past it then why not stop by to trade with all the other people from across the galaxy doing the same thing? And all those people stopping would attract entrepreneurs which would attract workers etc. Especially since in the fluff Absalom Station has pretty much hung out an "Open for Business" sign to the entire galaxy and isn't hostile to it at all (except for the Absalom Strong people but they're a minor faction).
I am applying the tendency of humans throughout human history to urbanize here but I think that's fair enough since humans in Starfinder are inspired by us Earth Humans. Plus the other core races don't seem averse to urbanization either (I'd say some like the Ysoki, welcome it).
Falling back to real world analogues, Absalom Station isn't just the political Brussles of the galaxy, it's also the religious Vatican *and* Mecca *and* Jerusalem, and the tourist Las Vegas, and the financial NYC, and the trading London, and the corporate Seattle, plus other fantastical and sci fi things that Earth cities don't have going for them.
And there's a lot of RPG setting cities that are meant to be big enough for entire Level 0 to Level 20 campaigns. Ptolus, Sigil, Many Pathfinder cities including Absalom itself in Golarian. I'd say the adventure hooks the Station Gazeteer provides in Dead Suns Vol 1 are enough to build a full adventure that might even culminate in Level 20 PCs trying to get past the Starstone defenses and to the artifact itself for one reason or the other. And I don't think that's a bad thing. I think that's a good thing.
I'll throw another reason why I really feel the Station should be bigger into the mix as well. The more people you have, the more diversity you can have, and diversity of alien species is one of the hallmarks of Starfinder. With a 46% human population and say... 20% more for all the rest of the legacy races (or even 10%) leaves less than a million souls for the other core races let alone all the other diversity of aliens that is a calling card of the setting and the mechanics of Starfinder. If you get to big city NYC level (8 million) then there's suddenly a lot more space for Vesk neighbourhoods, Android collectives, Ysoki clans, Lashunta enclaves, Shirren complexes, to *all exist* along with the Golarian remnants getting significant populations of their own at the same time. Get to megalopois level of population and so much the better (my preferred minimum of 40 million).
The basic thing here is that Abslaom Station isn't just one thing or the other. It's not just a religious center or a trading center or easy to get to or a cultural center of a planet. It's *all* of these things and more for a galaxy of species and not just humanity.
I will leave most of the above alone as I'd just be repeating myself. Just to note that Big Dumb Object is not meant perjoratively. It's just an old term for a certain trope of massive, mysterious objects in sci-fi (Roz Kaveney coined it).