Not the same thing. The Fed warp nacelles may have nothing between them, but think of it more as a flat wing that has been turned up at the end, like the little winglets we see on so many jetliners today.
I'm sorry, but that's just empty talk - by describing the Federation starship, you're still also describing the Romulan starship but without the useful add-ons atop the nacelles.
The outer flanks of the nacelles define the maximum width of the starship. Their aft ends or then some stern structure extending aft of them, together with the bow, define the maximum length. The top of the primary hull defines the top of the ship, and the bottom of the secondary one defines the bottom. Any volume left unused inside the box thus defined is wasted. And Federation designs waste more of it than this Romulan one.
The volume of the box is important in two ways. One, you have to pack all of it inside the warp field; two, you have to park all of it inside the starbase. It's irrelevant for these musts whether the volume consists of useful hull parts or empty vacuum. Yet one would think the organization running the starship would prefer things other than empty vacuum to equip the ship with.
The Federation ships aren't wasting space any more than jetliners are.
FWIW, jetliners waste space immensely
, and are hated for that. If the designers could somehow do away with the wings, they immediately would, as this would markedly simplify parking and taxiing the things. Shortening the wings would be a halfway solution; the lifting power could be shunted to correspondingly lengthened horizontal stabilizers. Alas, that's not fuel-efficient (although doing it and then combining the tips of the two sets of half-length wings in a diamond shape is
, and is being studied), and fuel efficiency is the one and only thing that trumps tarmac footprint issues in jetliner design.
Apparently, in starship design, keeping the space between the nacelles clear of obstructions is also a factor that trumps packing density...
Actually, it's the external dimensions that's the issue.
But this doesn't affect the shape
of the Romulan ship one iota. You can build that shape to be large or small, and still the efficiency issues are the same.
If the Romulans just wanted the ship to be wider than absolutely necessary, they are still
doing it in the most efficient way possible; doing it the Federation way, with just a single set of mostly horizontal pylons, would be more wasteful.
If, OTOH, the Romulans want to widen their ship without adding to her mass, then they are doing it all wrong; leaving out the top set of pylons would save mass. So frivolous widening of the ship probably isn't the driving force behind the ship after all, at least not in combination with mass saving ambitions.