Even their ships the Romulans went from BoP's to big War birds akin to D-7 cruisers and the Klingons went from big cruisers (actual D-7 cruisers) to BoB's.
Romulans never had Birds of Preys, from what appear to be window on the ship in Balance of Terror, that ship isn't exactly "small." There's no indication the Romulans ever had a warship the size if the Klingon smaller BoP.
And while Klingons did add the (actual) BoP to their inventory, they kept the D7 cruiser until DS9, and they also added at some point ships larger than the D7 cruiser.
What? Even Star Trek Memory Alpha refers to the Ship seen in the Balance of terror as a "bird of Prey." The ship literally has a a bird painted on the bottom. Now your right no exact numbers of crew are mentioned but the episode was based on a destroyer hunting down a submarine so its sort of suggested that the ship does not have a huge crew.