Why couldn't the Feds be copying the Roms?
We tend to assume the espionage, or even just the flow of information, goes only one way --radially outward from the Federation. I doubt this to be the case. Even if the notion of a compact saucer comes from the Romulan encounter by NX-01, it's still Romulan influence.
Of course, one could argue that the NX-01 was already a Miranda-like design.