In all the series, Romulans never demonstrated superior strength to humans, but at the same time, nothing seemed to contradict it, either. When Kirk Fu is involved, all bets are off, so I wouldn't just go by Enterprise Incident. The only other Romulan episodes I can think of where we see Romulans get into a physical fight is Unification 2, where they faced off against Picard, Data, and Spock, which is hard to gauge how strong Romulans are, physically.
One thing Star Trek has kind of dropped the ball on is that gravity is different on different sized planets. Is Romulus as massive as Vulcan? Or Bigger? Or smaller? Even though Romulans are an offshoot of Vulcans, until we know the answer to that, it's hard to say if Romulans are as strong as Vulcans. Also, if Romulans as a race never intermarried with other races, we don't know how much genetic drift occurred. Genetic drift, combined with lower Gs could make Romulans significantly weaker than Vulcans.
Also, are Romulans actually a hybrid race, and are not pure Vulcans, genetically speaking? For example Romulans are the product of their Vulcan ancestors, and another race (or races) said Vulcan ancestors intermarried with, and Romulans are the result, this could mean that Romulans have diverged to the point where they are physically weaker than Vulcans.
This leads to a good question: if the above is all true, why does Nero seem so much stronger than Kirk?
Now, if one were to consider "Star Trek: Countdown" as canon, this COULD explain Nero's strength, even if it doesn't explain Romulan strength levels. In Countdown, it is established that Nero uses captured Borg tech to modify the Narada. The Romulans could have used borg tech to modify themselves, and give them superior strength.