The Romulans seems to be able to get multiple large starships to locations more often than the Federation, while the Klingons tend to send multiple smaller ships. Starfleet usually only sends one ship to things, while the Romulans send two and the Klingons three. The difference being that the Romulan ships are each larger than the largest known Federation starship.
However when the war comes, the Romulan portion of the fleets seem smaller (in shots of the fleets, the map and charts seem to have forces as large as the Federation or Klingon fleets), though it is made up entirely of giant starships. They were able to hold there own against the Dominion once they entered the war, and their ships certainly held out better than the Cardassian ships during their little raid of the Changling homeworld prior to the war.