Romulans are necessarily weak(in numbers not technology) given the apparent small size of the Romulan Empire.
Romulus is visible on the star map in Balance of Terror and apparently also on the floor picture of the Senate in Nemesis.
Logically space travel would tend radiate outward from central point yet Romulus is fairly close to Federation space and Vulcan particularly.
Its possible that a vast swath of terriority has never been explained which lies in the direction of the Delta quadrant, but if that is the case I don't know which route all those El Aurian refugees took fleeing the borg.
Throw in other variables like long lifespan and presumably low Vulcan like rates of fertility and you have hard limits on how much space Romulus could have settled.
Also the top of the line 24th century Warbirds ran quite a bit slower than Federation starships so I doubt the Empire grew at a faster rate than the overall Federation from the Earth-Romulan war until "The Neutral Zone".