The outposts were constructed on asteroids but nothing say that the asteroids are in the same system. A series of star systems along the NZ, each with an outpost in it, for example. Which also provides an good explanation for the comet encountered if Outpost 4 was on the opposite side of its star when the BoP attacked. Passing through the inner system on the way home would allow it to encounter a comet with tail..
Why asteroids? Hanson's outpost was a mile under the surface; presumably as a deterrent for the primitive atomic weapons in use when it was established.
As for the size of Romulan space? Well, it does have to be large enough for an uncharted M-class planet, three hours from the NZ at some warp speed, to go unnoticed until some space hippies travel there.