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.
No, that's what it was defined as from the very beginning, in "Balance of Terror." That's what the term "neutral zone" always means -- a treaty-defined border zone between two hostile powers that neither of them controls and is therefore neutral (i.e. a demilitarized zone or buffer zone). BoT described it as "the neutral zone between planets Romulus and Remus and the rest of the galaxy," and the famous Neutral Zone map from that episode clearly shows the Neutral Zone as the thick dividing line between Earth space ("Federation" hadn't been coined yet) and the Romulan Star Empire. Nobody would call the empire itself "the Neutral Zone," because it isn't neutral.
What Duane and Peter Morwood actually wrote in The Romulan Way, p. 195 of the original edition:
The treaty established what came to be known as the Romulan Neutral Zone, an egg-shaped area of space about ninety lightyears long and forty wide, with 128 Trianguli at its center. The Zone itself was the "shell" of the egg, a buffer area all the way around, one lightyear thick, marked and guarded by defense/monitoring satellites of both sides. Everything inside the Zone was considered "the Romulan Star Empire," even though there was as yet no such thing.
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).
Well, it was Enterprise that canonically established the 22nd-century Romulans as a more advanced power than Earth. The Romulan War novels were following up on what the show established. And I doubt the show was following Duane's lead.