Curiously enough, we don't really even know whether the Federation is bigger than the Romulan Star Empire. There are no onscreen maps depicting the borders of these entities, not unambiguously, and there are no dialogue counts of star systems or other types of real estate.
That Romulans didn't send many ships to fight the Dominion doesn't mean they had fewer than the enemy. The British carefully avoided sending more aircraft to defend France when the situation there became hopeless in 1940; this should not be mistaken for meaning that there was a shortage of aircraft in Britain.
TNG - Galaxy Class> D'deridex Warbird
Doesn't seem like that when our heroes encounter those vessels. Sure, Romulans tend to send two to ensure a kill, but in TOS they sent three to five ships... One-on-one engagements are also typical, and at best Picard can hope for mutual destruction there.
Nemesis - Sovereign Class> Valdore Type Warbird
Based on what? The two never fought, or even postured against each other.
1. The Federation spans 8,000 light years. Simply put, it is at least twice the size of the Romulan Star Empire or the size of the Klingon and Romulan Empires put together!
2. Why would the Romulans only send a few ships to the front lines? The Dominion already outnumbered Starfleet, the Klingons and the Romulans combined. The Romulans would send most of their ships to the front lines because if they didn't win against the Dominion at the front lines, what good would their forces in their territory do against the Dominion. If the Dominion broke through and headed straight to Romulus, the Romulans would be destroyed.
3. The Sovereign class is obviously stronger than the Valdore type warbirds because of many reasons including armament. But in the battle against the Scimitar, those ships lasted what, like 5-6 minutes!
Meanwhile the Enterprise took far more hits and only two sections of their shields which they regenerated thanks to superior Federation technology.