About Romulus: IIRC, Duane's Rihannsu books painted a picture of a resource-poor Romulan Empire, made up of many conquered farming world's occupied by only a few thousand Romulans.
Many of the outer worlds were thinly populated, though the populations involved seemed to be on the scale of hundreds of thousands. Many others weren't: Artaleirh, in
The Empty Chair, had at least three major cities which reminded Kirk of San Francisco.
]But, the novels since have given us highly populated Romulan worlds.
This is a shift that occurred within Duane's own series.
The first two books described a Romulan empire only a century old, founded after the
Carrizal's contact with the Romulans in the Eisn system and the subsequent Romulan War. The Romulans settled two dozen worlds within their sphere--what they called the Empire, what the Federation called the Romulan Neutral Zone--but not all of these colony worlds were successful, and the Romulan state was still substantially smaller than either the Federation or the Klingon Empire.
This changed. The Neutral Zone, for starters, was identified in TNG as a buffer zone between the spaces of the Star Empire and the Federation. More, the Romulans were explicitly identified as being a power roughly on par with the Klingons or the Federation. In the second pair of books, this led to Duane shifting her description of the Empire, with characters distinguishing between first- and second-generation colonies ("overspill" colonies, she called them) and identifying client worlds (presumably conquered planets). The Empire's colonies were more substantial: Artaleirh was strong enough to lead a revolution in Romulan space, while the planet Ysail had been colonized for centuries (much longer than the Romulans' history of starfaring in the first two books).
This shift has led to a transformation in the depiction of the Romulans in the current universe. At one point, the Romulans were described as being substantially weaker than the Federation, at best trying to keep up, with a shorter history of starflight than the humans and many fewer and more fragile colonies. That's been reversed, most explicitly in the Romulan War novel: the Romulans have been starfaring much longer than humanity, and it takes multiple powers to keep the Romulans from conquering humanity (first the Haakonans in the rear area to keep the Romulans distracted, then an Andorian-Tellarite-Vulcan fleet at Cheron).