My somewhat tongue-in-cheek response was basing it on the pre-2012 D-18, since as was pointed out, the new D-18 is virtually the same as what was the defunct D-18V.
"Thin" relative to a good D-28 is and always has been my impression of the more modern, straight-braced, standard X position D-18. I owned a primo 1964, old factory D-18. It did everything a good D-18 is supposed to & did it well. But in the end, I sold it because it just sounded too thin relative to a good D-28.
The Authentics get beyond that (as do the pre-wars) because they have the meat, the bottom, that the D-28 is known/admired for, plus the cut the D-18 is known for. However, my contention is and has always been that a "good" D-28 does also have cut to go with the famous bottom end.
That's my story & I'm sticking to it!!!
P.S. I've owned one very good D-18, had longtime friends with old D-18's, D-18GE and D-18V, some of which have been in my hands for prolonged periods of time as I did work on them. I've played numerous D-18 Authentics. All of these have been in the context of being played A/B with my personal Yardstick of a guitar I've had for 42 years and am intimately familiar with. So even if I haven't A/B/C/D/E'd a vintage D-18, modern D-18, D-18V, D-18GE and D-18 Authentic, I'm still pretty comfortable talking about their relative merits & sounds comparatively without relying just on the memory of the guitar in the abstract.