This also has a lot to do with the speed of communication, speed and ability to transport material goods, and social homogeneity of its peoples.
The longer it takes to communicate, or transport goods, with the farthest reaches of your dominion, the more autonomous it naturally becomes. Greater autonomy means less reliance on the distant, central government.
Geography, bureaucracy, technology all help decide how fast communication crosses the empire.
Social Homogeneity...
There are a few modern examples showing that a socially mixed group fosters political factions: the EU, United States, Jerusalem.
As a political and financial collective of countries it is worth putting in this list because a collective of confederate countries is a political pathway toward a unified political system. However, the EU is having trouble maintaining cohesion because of member countries like Greece. Greece has a separate social, political and financial approach when compared to countries like Germany. Greece isn't only financially unable to support itself, but often times seems socially unable to right its own financial practices. This is hard on Greece, AND it threatens the strength of the EU because agreed rules and practices are not being met.
The United States is an excellent candidate when discussing political unrest due to a socially heterogeneous collective. To this day, the country faces ideological fighting from its civil war: the systemic racism in politics and government, the growing animosity in a political discourse that has no apparent reason for it, and the fight over a Southern flag and what it means even 150 years later. While it's easy to say that America doesn't fit in this discussion because it's still a unified country, the amount of political rhetoric about various states seceding from the Union gives a non-trivial hint that all is not well - despite the country having suffered worse. A look at the origins of the US will also show that it was formed because a few key conditions were met. The time to communicate with the central government was so slow that the Crown purposefully established governors to act on its behalf. A few generations in and all the governors were now local born citizens of colonies who were more aware of the troubles of the colonists than of the Crown. They were rooted in the community, not the Crown.
The ability to ship goods was slow, and therefore expensive, this forced the colonists to become self-reliant, again making the generations of colonists more aware of their problems as farmers and providers, then of the problems of the the distant and absent economic presence of the Crown. The colonists were allowed to become more self-sufficient because of the access to material goods at their hands. This unique circumstance also worked against the strength of the Crown.
Jerusalem has, for thousands of years, been a land of contested ownership, with the major Abrahamic religions warring each other over control. Even now, the city is split, like Berlin was after WW2. Like Jerusalem, Berlin (and therefore Germany) was controlled by political groups with cross purposes. In fact, I think we can say that much of the Middle East is at a constant war whether the war come in the form of political, martial, ideological or financial methods. With these wars, and these factious beliefs, there can be no unifying political or social body.
Speed of Communication and Transport...
The size of the known world, and the speed of light, both relate to a political entity's communication. These also relate to technology, geography and bureaucracy.
There was a time when the peoples Europe and Asia believed that the world ended with them. To explore meant to send ships, if they had ships. Simply, there was a time before ships which greatly impeded travel which necessarily impeded communication and transportation. Fine, so ships came along. But they weren't really great ships at first, and definitely not technologically sound enough to travel the oceans and do so reliably. How could anyone navigate when the clouds hid the night stars? How could they accurately tell time, to measure distance, even if the stars were out? These missing technologies caused many adventurers to go missing therefore making communication and transportation unreliable.
So, fast forward a few hundred years. Now, we have accurate naval time pieces and GPS and satellites and cell phones and such. All this means is that we are better able to colonize farther reaches. But, we're still hindered by the very fabric of the universe itself. If we were to talk on the phone and experience a delay between speaker and listener, of more than a half second then we would become irritated therefore making it less likely that we would talk. It would take effort and patience and live communication wouldn't be spontaneous. It would become a series of reports, or a Q&A, and a pre-planned one. This delay in communication only gets worse as we start to imagine an empire that spans neighbors in a single solar system. Imagine a half hour delay between speaker and listener as we talked to a friend on the nearby planet. Now, imagine running a full parliamentary procedure that involved a teleconference with that neighboring colony. To be sure, that neighboring colony would need a political figurehead, like a governor, to oversee the daily operations of the business of the pan solar collective. Of course, now the local figureheads and colonies are being discussed, we come back to a very old problem in social segmentation... autonomy via self-reliance and self-identification (ie. Us vs them). Technologically, how do you slow this erosion of political influence? Increase technology or keep like minded colonists on your planets and rotate them out occasionally.
