Regarding the size of the Federation versus the size of the Romulan Star Empire, and how it relates to the original question, I do subscribe to the notion that the Federation is more "nebulous" with non-member systems packed between member systems, making a Federation star chart look vaguely like Swiss cheese. Would those non-member worlds be considered in Federation space, as least as protectorates or something? And what would happen if one decided to join another alliance? The size of the Federation is probably not accurate to its true strength, given that we can be fairly certain of its sprawl.
The Romulan Empire, on the other hand, would be one primarily of colonization and annexation, therefore it would likely be more tightly packed and have a smaller border. This would also fit well with the Romulans being a generally xenophobic people; having a tighter border would make entrance and egress to their space easier to control. That being said, I think the Empire's size is also not an accurate representation of its power. Some have speculated the Empire to be much like ancient Rome - overcrowded, underfunded, and generally in need of MORE of everything. I would counter that surely an Empire capable of mastering the apparently complex nature of an artificial singularity power source is not deprived of key resources, at least. Oppressive? So it seems. Impoverished? I doubt it. The Romulans do like to make a show of things, and surely their massive warbirds are thus designed largely for this effect, and perhaps larger than they "should" have built, but I doubt that they are starving their children to do so.