But are the Romulan colonies puny, or are the population and resources more evenly distributed? I don't think there's an answer in canon, but I picture it as reasonably well distributed. The Romulans were powerful enough to turn the tide in the Dominion War and too powerful for the Federation and the Klingon Empire to deliberately pick a fight with or conquer. How would they maintain a large empire without a great many powerful/resource rich planets? I don't see all of that power coming from one planet with a few puny colonies.
Also, taking out Romulus might lead to the government relocating - and being attacked en route by a dozen different factions wanting to be the next government.
Would this be much more dangerous than usual? They probably guard against assassination on a daily basis, being the paranoid people they are. And do we know that there are a dozen powerful opposing factions just waiting for an opportunity to pounce? I'd think the Romulans would pull together in the face of a crisis. They're xenophobes - their first priority would be maintaining their borders and not looking like a target to their neighbors.
At the end of the week, there would be sixteen new capital planets and fifty-eight separate warfleets of changing allegiances
If this were the Klingon Empire, sure. A hundred different clans would battle it out. But the Romulans always seemed more united than that. They had their problems, but they didn't give the impression that the empire was on the verge of collapse. Quite the opposite, actually.
But if you're starting out with the "puny colonies" viewpoint, we're not going to agree. Your argument does make sense if the colonies are small.
From Memory Alpha:
Benzar (taken in 2374, likely returned to the Federation)
Carraya IV (Carraya system) (likely outside of actual Romulan territory)
"Eden" (likely uninhabited)
Khitomer (within Romulan territory as of 2369)
Remus (Romulan system)
Romulus (Romulan system)
Unnamed Minshara class planet
Surprisingly few Romulan planets have been mentioned. Many, many more Klingons plants have been seen or mentioned, Even the supposedly resource poor Cardassians have a list many times longer than the Romulans. Maybe there's something in the Romulan psyche that doesn't like colonizing. Like the Jews wandering the the desert for 40 years, once they settled somewhere they were in no mood to budge. A few slave races could do all their mining, food production, etc. with a token Romulan garrison to keep the peace. Add in the Romulan arrogance seen in STXI where they didn't believe Spock (Nah, it couldn't happen to us, we're Romulans!) and I could see them having few colnies to carry on. More than we've heard of but fewer than the other major empires. And let's not forget that everything between Romulus and the Hobus supernova is toast as well.