Rome understood the concept of mobility, which is why they built roads throughout their empire. England accomplished its blobbing primarily through naval transport. The railroad helped cement both the US and the SU as stable and internally supportable countries, rather than fracturing into smaller defensible realms. It's not the only mechanism, but plays a part.
It's a technological limitation to which both England and Rome concentrated on sollutions in so far as they had the know how. Lets just say that Rome and England also knew the importance of heavy investment to maximise the amount of territory they could cover in a reasonable time. I see it as a "hard limit" depending on technological advancement and it's observable imho how size of empires increases as technoligy evolves.
It takes a strong army to put down the inevitable tax revolts, and covering the exorbitant costs of supplying and paying the huge army necessary to cover the long hostile borders eventually becomes the primary goal of the state. Assyria fell into that trap, and Rome did pretty much the same. Both fell into to civil wars, army revolts, and then outside forces attacked the weakened remnants.
The Romans knew it when they were hitting the limitations of their empire size and retreated from area's like Dacia later for example to have a more defensive position behind the Rhine. Perhaps Rome also could and even should have kept Messopotamia but it was given up iirc for similar reasons of "becoming too large". It was noticable how difficult it was to defend the border given the possibillety of concentrated enemy forces at a certain point and the apparent need to draw men away from defensive positions all over the empire to shore up an area at many times.
Another factor that might be easily overlooked imho in millitary terms is the command of imperial armies. Certaintly for Rome it's apparent that any general could become a potential rival for emperor if the emperor would not take to the field himself to defend the Empire as so often needed, yet the emperor could not be everywhere and the later addoption of the tetrarchy also showed that need to have sufficient trustworthy generals to defend the many wide flanks of the empire. The larger a empire grows the more seperate armies it requires and the thus trustworthy and capable people to lead them, afaik trough much of history this was a big issue as giving command of a big army could play into the hands of any ambitious would-be warlord, and at times the soldiers would even make that decission for the general.