Me: "The idea that there wouldn't be large, populous, Romulan colonies, strikes me as unlucky. As much as the humans, the Romulans are an expansionistic culture, and the idea that in a history of starlight several times' longer than humanity's they wouldn't have founded a substantial number of populous colonies strikes me as unlikely. There's plenty of unpopulated worlds suitable for colonization in Federation space. Why wouldn't that also be true in Romulan space? And why wouldn't the racist Romulans preempt the expansion of subject species (well, maybe not as labourers) onto these worlds?"
Christopher: "Indeed. Let's keep in mind what the word "empire" actually means. It doesn't mean one monocultural state that's mean to its neighbors. An empire, by definition, is a multicultural political entity in which one central state (the metropolis) governs multiple other states and harnesses their wealth, resources, and labor for the benefit of the metropolis. The Romulan state is called a Star Empire, which explicitly characterizes it as a power that rules over multiple star systems, presumably including ones that are politically and culturally (and, given the context, taxonomically) distinct from the people of Romulus."
Most of the traditional empires of Earth contained colonies of settlement and colonies of natives: Britain had the Thirteen Colonies and Bengal in the 18th century, say. Over time, as empires and populations expanded, these boundaries blurred, their categorizations changed. In the 1860's the territory of Rupert's Land
--basically, most of what's now central and much of Arctic Canada--was populated mainly by First Nations people. The demography's much different now. And where do Indian immigrants to the Caribbean and Fiji--colonial subjects heading to another colony--fit in the settlement/native colony continuum?
The only colonizing nation I know of that didn't acquire and produce colonies of settlement alongside colonies of natives was France. That was produced in the first empire (to the Revolution) by a disinterest in overseas interests and a desire to profit from extractive resources like furs and sugar. Combining mass settlement with these extractive resources could have jeopardized the empire--if Canada had been colonized heavily, apart from agriculture wrecking the forests that were home to fur-bearing animals the natives who were the most reliable allies and trading partners of the French would have been upset. In the second empire (after the Revolution) by the lack of any significant interest in emigration and, perhaps, an interest in propagating French influence through popular culture more than through overseas migration, Algeria being the single large exception that proves the rule and even then most of the immigrants came from non-French Mediterranean Europe.
Those motives explain two centuries of French non-colonization. The Romulan Star Empire, according to Memory Beta, dates from the 3rd century CE
. The RSE probably did have its periods of static growth, periods of disinterest in the settlement of new worlds to the profit of conquering already-inhabited worlds. I can't see pressures towards non-settlement enduring throughout Romulan history, especially given the apparent Romulan interest in physical footprints and contempt for subject species. How can Romulans have an empire if there aren't enough Romulans to securely rule it?