Not necessarily
There are some considerations, originating from various aspects, that can be made about the possible efficiency of this kind of design.
Engineering
According to Roddeberry guidelines about starship design, warp nacelles should be visible to each other; this require them to be positioned outside the main hull, and having relatively small/thin pylons means using less materials and requiring less systems; on a big project like a starship, you try to economize resources, if you can use let's say 10% less resources by not adding more than is required, it means that you can field one more ship every ten.
Even if the nacelle visibility rule seems to be mostly valid for Federation ships, it is likely that even other Empires' technologies could have similar positioning requirements and that the overall design is affected by the functional aspects of the ship systems.
Armor
The thickness of a single structure, like the wings or the nacelle pylons, is not the real factor to be taken into account; the real important thickness here is that of the hull/armor, instead of the whole structure: in example, consider that a big portion of the secondary hull of a Galaxy-class ship is occupied by the deuterium tank, which lies just below the hull; a direct hit opening an hole in the hull could potentially do more damage here rather than on a warp pylon, even if the "shape" of the ship is more preserved; this reasoning of course is valid also for other ship classes, not just for the Galaxy.
Shields
Romulan ships, like all other ships in Star Trek series, are equipped with shields, which serve as the primary defense against attacks towards the ship itself. This reduces the need of structural strength and thus of "bulky" ships, because if you count the total firepower of an enemy as 100%, a large percentage of this damage is virtually absorbed by the shields.
Cloaking
Romulan ships are equipped also with cloaking devices; if the battle won't go well, let's say your ships are almost depleted, a viable strategy could be to just hide and adopt a more cautious approach to the battle, maybe made of rapid hit-run-hide flybys rather than a more direct tactic based on heavy artillery fire; in fact, given the general Romulan attitude, based on subterfuge, a sneakier approach seems reasonable and even desirable if you possess the technology to do so.
Maneuvering
Probably one of the most important aspects, having a ship with thin structures could also be a tactical advantage. If you offer to your enemy the least wide profile of your ship, let's say the frontal one (like the cited Romulan ships), you give your enemy far less surface to be targeted; one could argue that the enemy don't stand still and will try to outmaneuver you, but so do you; in the end having a thin profile is potentially always a problem for the enemy and a possible advantage for you. While this is true on one-to one battles, it will become even more relevant when two fleets in a line or wall formation are facing, since you must outflank an entire formation and not a single ship.
it is basically the same principle of aerodynamics, like in the wing shape of modern planes, but instead to offer the least possible surface to air to avoid friction, you offer the least possible surface to enemy to avoid weapons.
To use one of your examples, a Borg cube can't use this kind of tactic, because it don't have a profile that offer the least possible surface and can be easily targeted from every position; so in this respect being bulkier is a disadvantage, not an advantage.
Other Empires' Ships
Even other Empires use ship designs which feature small/thin structures, like the aforementioned Federation warp nacelles; but also Klingon and Cardassian ships, just to cite two prominent examples, have designs with relatively thin structures if compared to the general shape of the ship/starbase itself.
Being so evenly spread, it seem very likely that this kind of design, in this fictional universe, don't constitute a great problem in the end, so there is no reason to suppose than a single Empire, the Romulans in this case, should not adopt these principles.
And of course, all these considerations must face one fundamental truth: no matter how strong your ship is, if the plot need it to be damaged or destroyed, it will.