Well, the implication in Balance of Terror was that after the war the Romulans were supposed to be sealed up within their own Solar System. Their "Empire" was just supposed to have been their two worlds of Romulus and Remus, so maybe part of the Treaty was them giving up most of their Slave Soldiers.
Obviously the writers changed their minds later on about how big the Romulan Empire was.
No they changed their minds during writing and rewriting of "Balance of Terror".
Captain's Log, stardate 1709.2. Patrolling outposts guarding the neutral zone between planets Romulus and Remus and the rest of the galaxy, received emergency call from outpost 4. The U.S.S. Enterprise is moving to investigate and assist.
Clearly the planets Romulus and Remus and their star system or systems are surrounded by the Romulan Neutral Zone.
SPOCK: Which allowed no quarter, no captives. Nor was there even ship-to-ship visual communication. Therefore, no human, Romulan, or ally has ever seen the other. Earth believes the Romulans to be warlike, cruel, treacherous, and only the Romulans know what they think of Earth. The treaty, set by sub-space radio, established this Neutral Zone, entry into which by either side, would constitute an act of war. The treaty has been unbroken since that time. Captain.
The Romulans have never sent even a single spaceship across the Neutral Zone (or at least been detected doing so) and so can not crush rebellions on any planets beyond the Neutral Zone. Thus they must rule only planets in the one (or more) star systems within the Neutral Zone around planets Romulus and Remus.
But:
CENTURION: We've seen a hundred campaigns together, and still I do not understand you.
So two members of a Romulan stars ship crew have fought a hundred campaigns together. Maybe they fought as Romulan space marines on the ground of planets, but more likely they fought aboard Romulan star ships. Perhaps the Romulan space navy fought a hundred campaigns in a single war against a large and powerful space government. Maybe they conquered a hundred separate space travelling realms as powerful as the Gorns or Cardassians or First Federation in a hundred separate space wars.
If the Romulan star ships can't travel across the Neutral Zone, wouldn't they have to reach distant stars by using stable wormholes or something to disappear from one place and reappear in another distant star system? And form another neutral zone around it after conquering it?
In "Balance of Terror" Kirk says:
KIRK: Open a channel to our nearest command base. Quarter hour reports on our position and status.
UHURA: Yes, sir.
Later:
KIRK: Are you continuing to broadcast tactical reports?
UHURA [OC]: Affirmative, Captain.
KIRK: And at this distance?
UHURA [OC]: Approximately three hours before receiving a reply to our first message.
Thus the subspace radio round trip time to the nearest command base should be at least three hours.
And later:
KIRK: How many men did we lose, Bones?
MCCOY: Only one. Tomlinson. The boy who was getting married this morning. His fiancée is at the chapel.
(Kirk turns to go, and Rand enters)
RAND: We finally received an answer from Command base, sir. They say they'll support whatever decision you have to make.
Thus the subspace radio round trip time to the nearest command base should be somewhere between three and twenty four hours.
In "The Enterprise Incident" in another part of the Romulan Neutral Zone:
KIRK: You understand that Starfleet Command has been advised of the situation?
TAL [on viewscreen]: The subspace message will take three weeks to reach Starfleet. The decision is yours, Captain. One hour.
Three weeks for a one way message should be between 7 and 21 days, and thus 168 and 504 hours. Thus if the speed of subspace radio is the same the distance between this part of the Neutral Zone and Starfleet Command, presumably on Earth, should be 7 to 168 times as great as the distance between the part of the Neutral Zone in "Balance of Terror" and the nearest command base.
In "The Alternative Factor" Kirk has a conversation with no time lag with Commodore Barstow at Starfleet Command, presumably calling from Earth.
KIRK; Aye, aye, sir. Can you assign me other starships as a reserve?
BARSTOW [on viewscreen]: Negative. I'm evacuating all Starfleet units and personnel within a hundred parsecs of your position. It's going to be tough on you and the Enterprise, but that's the job you've drawn. You're on your own.
A hundred parsecs is 326.156 light years. Presumably Barstow is not evacuating starships from Earth and the inner systems of the Federation, and thus Kirk should be more than 326 light years from Earth. If the time lag for subspace radio waves is less than 3 seconds at a distance of at least 330 light years, the section of the Romulan Neutral Zone in "Balance of Terror" should be at least 1,782,000 to 28,512,000 light years from Earth and the section of the Romulan Neutral Zone in "The Enterprise Incident" should be at least 19,958,400 to 59,875,200 light years from Earth.
Since our galaxy is only about 100,000 to 120,000 light years in diameter, those figures don't add up. But they imply that the Romulan Neutral Zone is spread out over a vast distance.
In "Whom Gods Destroy":
SPOCK: Fascinating. What maneuver did we use to defeat the Romulan vessel near Tau Ceti?
KIRK 1: Very good, Spock. The Cochrane deceleration.
KIRK 2: Spock, you know the Cochrane deceleration's a classic battle maneuver. Every Starship Captain knows that.
So a Romulan starship was destroyed, possibly by the
Enterprise, and possibly near the Romulan Neutral zone, near Tau Ceti, which is about 11.905 light years from Earth.
In "The Deadly Years" the
Enterprise makes an ill advised shortcut from Gamma Hydra across the Romulan Neutral Zone to Starbase Ten. Gamma Hydra is presumed to be Gamma Hydrae, about 138.8 light years from Earth - about 11.658 times as far as Tau Ceti. The angles and times during the journey give the impression that the Romulan Neutral zone is curved and much smaller than the distance from Tau Ceti to Gamma Hydrae.
The evidence indicates that the Romulans travel through wormholes or something to other star systems far beyond the Neutral Zone and conquer them and create new Neutral Zones around them. Thus the Romulan Empire should consist of tens or hundreds or thousands of separate sections, each containing one or more star systems and each surrounded by a section of Neutral Zone.