A few interesting details from story outline for "Balance of Terror":
...in the direction of the twin giants Romulus and Remus 638...
This description was shortened to just "...
twin giants Romulus and Remus" in the first draft dated 6/21/1966.
The script describes the map as showing:
...a curved space marked "Neutral Zone", and extending off the map beyond that a star system of which one of the planets is marked "ROMULUS"...
The full quote describing the map, from the revised final draft dated 7/18/1966, is:
Beyond this curve, a curved space marked "Neutral Zone", and extending off the map beyond that, a star system of which one of the planets is marked "ROMULUS"... the entire area marked "ROMULAN STAR EMPIRE".
My interpretation of this, and I'm recognizing that Roddenberry encouraged his department heads to be creative in translating script directions, (i.e., as evidenced by his coinage of the term, "measure," per his memo of 6/15/1966), is:
1. "...
and extending off the map beyond that..." implies that we only need to see the relevant portion of the map and not the whole thing. Indeed, the sun of the Romulan/Reman system can be off the map entirely if it's not needed.
2. "...
of which one of the planets is marked "ROMULUS"... " This seems to indicate that one of the planets on the map needs to be Romulus and, via what the ellipses suggest, the artist can draw another planet (or planets) on the map... and name one of them "Romii," if they wish.
3. "
... the entire area marked "ROMULAN STAR EMPIRE"." This portion of the sentence implies that the area beyond the "Neutral Zone" should be labeled "ROMULAN STAR EMPIRE."
4. Finally, in looking at the film of the episode in high resolution, Romulus, as shown on the map on the bridge main viewing screen, is actually a small solid circle with a circle drawn around it. I suspect that the artist was merely trying to draw attention to that particular celestial object (i.e., by drawing a circle around it) in order for the viewer to find it more easily. Otherwise, the solid circles of Romulus and Romii are exactly the same diameter.