There's likely an optimum education vs. population level required to advance a polity's tech level (and thus world dominance). A large population that's poorly educated will actually be a drag on advancement, since you more-or-less have to advance the whole population at the same time. While you can boost the tech level of an elite in such a polity by importing the relevant tech, by definition this means that polity isn't on the cutting edge of development for that world. If you lack the educational infrastructure the next tech level (e.g. if most of your population are functionally peasants), you aren't getting anywhere fast.