I have another theory, and there seems to be at least some basis for it historically.
One of the peculiarities of the Jews is their insistence that all adult men, or at least as many as possible, attend school and learn to read their scriptures. It's their duty to become literate so they can read the scriptures to their children. This is traditional and not always observed, but it did lead to a larger than usual educated Jewish middle class that was literate and numerate.
Enter the Roman Empire. A massive, unwieldy mammoth run by a people with a very good idea of how to conquer, but much less of an idea how to administer their conquests and in need of a massive influx of literate administrators and bureaucrats.
This created a massive opportunity for the literati of subject races, especially the Greeks who the Romans already culturally respected and borrowed heavily from, but the Jews as well, as another highly literate people, would have been in position to take advantage.
Basically it's not inaccurate to say that the Romans conquered and governed their Empire, but the Jews and Greeks ran it, in the sense that these people featured heavily in the rank and file bureaucracy of government, simply because they were a large part of the available labor pool of highly literate administrators.
They would hardly be the first Empire to experience this phenomenon. Jewish ex-pats rose to some prominence in both the Babylonian and Persian empires, and if the Bible is to be believed, the Egyptian as well. Not to mention that this is how the Han Chinese retained their cultural identity and ultimately converted their conquerors time and again.
In other words, create a large more-or-less cosmopolitan nation including Jews, and they'll find a way to make themselves useful to its leaders. This, of course, will lead both to opportunities for comfortable living, and the consequent increase in relative population that a disproportionate share of the middle and upper class population would mean.