One would assume the original treaty between Earth/The UFP and Romulus allowed for the latter's reasonable growth. Eventually, though, they would have begun to chafe behind an absolute boundary.
That might depend on which side won, and by how much. Perhaps the Romulans were really squeezed in tight because the Feds or Earthlings thought that this would be the best way to render them harmless. Angry, perhaps, but harmless.
The Romulan Commander and Centurion speak of having shared many a campaign. Might be Romulans fighting amongst themselves on the surface of a single planet; might be evidence that there is room for interstellar warfare within the Romulan territory. (Or perhaps the two codgers refer to the good old days before the Zone was created?)
Later on, in TNG "The Defector", Admiral Jarok says he has seen hundreds of planets, while by the strict interpretation of the treaty he couldn't have stepped outside the RNZ. Either this means the Zone (at least in the TNG era) encompasses hundreds of planets, or then Jarok simply went on illegal business a lot.
Perhaps the Treaty of Algeron permitted Romulan expansion away from Federation and Klingon space, but retained the Neutral Zone impinging on those two states to curtail the opportunity for further 'incidents.'
Or then the Zone was created between Romulan and Federation holdings, and the Klingons later expanded to rub against it - after which the stretch that lay between Romulans and Klingons became meaningless because the Feds could not enforce it and the other two players had every reason to disregard the old border and try to establish a new one more to their liking.
(To nitpick, the jury is still out on whether the Zone was established by the Treaty of Algeron, or by some other treaty. All we know from "The
Pegasus" is that Algeron bans cloaking and has kept the peace since the early 24th century. And both of these attributes actually speak against Algeron being the original treaty, because the original one was from the 22nd rather than the 24th century, and there shouldn't have been cloaks in existence back then, and thus no reason to ban them.)
Worf also says, in "Tin Man," "The Romulans claim all in their field of vision." The clear implication: Their empire is expansionist, and expanding.
A very good point. Worf might refer to what takes place between the Romulans and Klingons on their shared border, though, without implying violation of the Romulan/Fed border treaties.
As for "claiming all within vision", this might be how the original Fed/Rom treaty was formulated, too. Perhaps the Zone actually arrogantly bisected the galaxy so that Feds got one half, Romulans the other? Within those halves, the players could do as they pleased, but they couldn't interfere with the other half. However, such an arrangement would be impossible to enforce with asteroid fortresses... And "Balance of Terror" seems to suggest that the treaty truly was being enforced, and wasn't merely a dead letter.
In the end, different writers of Romulan episodes obviously thought differently about the treaty and the Zone. But then again, we know from episodes like "The Defector" that the Romulans themselves change their thinking more often than they change underwear. Perhaps there are alternating policies of blatant defiance and meek submission, and the Feds tolerate the former because the Romulans generally learn to prefer the latter when their expeditions to the Outside turn out disastrously.
Timo Saloniemi