As far as resource bases go I've never seen any one race own a "preponderance" of them but I'm not sure what is considered”preponderance" as far as base ownership goes.
By that I meant if a particular AI had 4 or 5 econ resources for something like a total 200% econ bonus then that would be an obvious reason for that AI to have an economy greater than any other AI.
If I look at the graph at the end of game they're stats are 5x more than anyone else's for no apparent reason.
Perhaps this is just indicitive of the best AI in the game. For example in every game I play my stats are certainly higher than any AI's stats in *every* category, but there's nothing mysterious about it. I have the best stats in everything because I have the most planets and therefore it's only natural that I have the highest pop, the highest income, the highest military, etc. etc.
If you're saying that every game a different AI ends up the strongest AI by a significant margin then I see nothing unusual. This just indiciates that in one particular game one AI happened to outcolonize the others for whatever the reason.
If you're saying that it's always the same AI the ends up in this position then I would simply guess that that particular AI is the strongest, at least based on the game settings (i.e. galaxy size, number of opponents, planet count) that you normally select.
However if you're saying that you have one AI in the game that has significantly higher stats than all the other AI's in the game and that AI has no noticeable advantage in pop, planet count, mining resources, super ability or any other reason that would otherwise explain the difference then either you've found some bug or there's some difference that you're not noticing.
The thing is that you're not supplying enough information by which to make this determination other than saying "they're stats are 5x more than anyone else's for no apparent reason."
There's always a reason if you dig deep enough but without the explicit details no one can really hazard a guess.