If you really think about it then a long distance scan of a 22nd century Earth ship might only show something like a saucer or a relatively thin flat like hull with nacelles. If that is the kind of design used by Earth ships to combat the Romulans then it's just conceivable they might copy the design to some extent.
That idea doesn't hold up if you go by ENT that showed Romulan ships of the era that also look more advanced than the TOS BoP. The ENT ships looked more like TMP era style Romulan ships.
It's also conceivable that while they didn't allow themselves to be captured Romulans might have captured some Earth ships and thus had gotten some technical ideas from them.
That idea doesn't hold up if you go by ENT that showed Romulan ships of the era that also look more advanced than the TOS BoP. The ENT ships looked more like TMP era style Romulan ships.
It's also conceivable that while they didn't allow themselves to be captured Romulans might have captured some Earth ships and thus had gotten some technical ideas from them.