The Federation is the strongest power in the Milky Way behind the Dominion, Borg, and Species 8472 (or the Undie). It has more ships than the Romulan Empire and more powerful ships to say the least.
Plus, remember at the beginning of Star Trek Nemesis when the Romulan is trying to convince the senate to join Shinzon. He says that with Shinzon, not even the Federation would be able to stand in their way. This indicates that the Federation is stronger than the Romulan Star Empire.
"1. The Tal Shiar fleet was destroyed more than two years before the outbreak of the Dominion War."
In the DS9 episode where the Tal Shiar and Obsidian Order Fleets are destroyed. The changeling on one of the Romulan ships says that with the Obsidian Order and Tal Shiar fleet destroyed, the only real threat is the Federation and the Klingons. This means that the Romulans took heavy losses to their fleet. An 2 years isn't a long enough time to rebuild a fleet.