I'm talking about the ship wasting a lot of space with its current shape and size.
But then you are wrong from the get-go, because wasting of space through shape is a relative measure and cannot be anything else.
Sure, it would be wasteful to build a big ship when a small one will do. But the shape isn't much of an issue in that case. And the shape of the Romulan ship is not inherently wasteful; the E-D is more wasteful.
I disagree with this because even with a single set of mostly horizontal pylons, it would still be wasteful for a ship to have big giant wings just for the sake of having big giant wings.
Wings are payload. Wings are useful volume. Any lack of wings is empty vacuum, which is not payload and not useful. A ship shaped like the E-D hauls empty vacuum where it could haul payload.
efficient, then the best thing would be to build starships without any hulls whatsoever, merely consisting of a pair of engines mounted as far away from each other as possible. Plus possibly a wireframe encompassing a really big box of vacuum. But the Romulans go for maximal hull, rather than maximal vacuum. The Feds go for minimal hull and maximal vacuum. Whether they go for fugu-style bloated-scariness factor as well is an unrelated issue...