you're young until you're not

In both of the interviews linked above, every mention of Hux is accompanied by a mention of “extreme” or “severe” silhouettes. This is noteworthy because Domhnall Gleeson—and therefore General Hux—has a very narrow build. He’s not particularly physically imposing, despite his height, which means Kaplan made the choice to simulate mass on Hux where there is none, seen most obviously in the breadth of his shoulders. The tailoring details of his uniform (e.g. princess seams, padded shoulders, and belted natural waist) are designed to exaggerate Hux’s shoulder-to-waist ratio by creating an inverted-triangle shape, and his greatcoat adds to the illusion because the tapering of its frankly massive lapels follows the same lines.

[LEFT: Domhnall Gleeson on the red carpet. MIDDLE: General Hux and Kylo Ren on the bridge, TFA. RIGHT: General Hux on the bridge, TLJ.]

This information may originate from our side of the fourth wall, but that doesn’t preclude it having an in-universe explanation. Kaplan states in one of the interviews linked above that Hux’s costume “is unique to him,” which, given its similarity in broad strokes to the costumes of other First Order officers, implies a uniqueness in its tailoring rather than its concept. We can therefore infer that said tailoring—and the illusion it creates—is a choice made by the character. Hux may not be able to do much about his natural slenderness, but he can manipulate others’ perception of it; this is very much in keeping with the canonical material we do have about Hux’s approach to his appearance.

From The Force Awakens Visual Dictionary:

Hux has always believed that appearances are vital for maintaining discipline, and wears a parade uniform designed to broadcast his authority as general.

Precision tailored military uniform evokes Imperial imagery

From The Rise of Skywalker Visual Dictionary:

Hux hates the Knights of Ren; he despises their unkempt appearance and how they tread dirt into the polished halls of First Order capital ships.

Hair coiffed to Hux’s exacting standards

The Star Wars visual dictionaries paint Hux as a character deeply concerned with his image, because he associates appearances with authority and discipline. We see this philosophy in action in The Last Jedi, when Hux is Force-slammed into his own bridge by Snoke via holocall; beyond an initial surprised yelp, he barely reacts to the violence, and the next time we see him in Snoke’s throne room, he’s washed the blood from his face and slicked his hair back within an inch of its life. Hux reacts to a reminder of his own vulnerability by controlling what he can control—and in this case, that means the way he looks.

TEXTUAL

I assume we’re all familiar with the “thin as a slip of paper and just as useless” quote from Brendol in Aftermath: Life Debt. This line is only the tip of the iceberg, as the insults Brendol is willing to voice in front of his superior officers are almost certainly tamer than those he’s willing to hurl in private. Though Grand Admiral Rae Sloane orders the Commandant to stop hurting his son in the following novel, Aftermath: Empire’s End, she has no way to enforce this ban on verbal abuse. (Or physical abuse, if we’re being realistic.) Said abuse is a near-constant reminder of Armitage’s physical “deficiencies”—likely due, at least in part, to chronic malnutrition during the early years of the First Order. Armitage’s resulting insecurities are reinforced through adulthood by the stark physical contrast between himself and Captain Cardinal, the human tank of a stormtrooper whom his father makes no secret of preferring.

We can also pretty reliably identify a character’s insecurities by observing what they envy in other people, and in the case of Armitage Hux, he (begrudgingly) envies both of his rivals—Cardinal and Kylo Ren—their physical prowess.

From The Rise of Skywalker novelization (emphasis mine):

"He's almost beautiful to watch," mused Allegiant General Pryde, standing tall beside him. The older man had arrogant blue eyes and a high hairline that seemed immune to perspiration, even in a hell-climate like this. "Don't you think?"

Hux refused to gratify that with a response, because true beauty came from discipline, from order. So it was almost against his will that he found himself mesmerized as Ren met a barbarian's charge head-on, cloak flowing, mist swirling around him. The glow of his lightsaber occasionally snagged on his cheek scar, making it appear as though a crack of glowing lava slashed his face. It was like something out of a dream, or maybe a nightmare, as the Supreme Leader plunged his fiery crossguard into his attacker's abdomen, lifted him from the ground, and sent him toppling onto his back.

Hux has very little good to say about Ren in any Star Wars text, but here he’s “mesmerized” “almost against his will” by the sight of Ren using his considerable strength to hurl Sith cultists around like sacks of flour. This is not a scene wherein Ren uses the Force to do battle, so the focus is on his physicality alone. Given that Hux is the creator of the First Order’s various combat training simulations, one has to wonder how many times a younger Armitage might have watched Cardinal use his strength with the same reluctant fascination.

There’s nuance here, of course. Hux’s hyperawareness of his appearance and the physical “weakness” it might betray doesn’t necessarily translate to self-loathing. I think Hux is perceptive enough to understand that his body—tall and slender with unique coloring—is advantageous in other ways; while it may not be the ideal build for a warrior, it has its own elegance and aesthetic appeal.

From The Last Jedi novelization (emphasis mine):

The Finalizer's gleaming black bridge was a model of efficiency, with controllers and monitors briskly exchanging information from the Star Destrover's targeting computers and sensor suites. Hux smiled at the thought of himself as the center of all that activity—a slim, dignified figure in black, uniform perfect, standing at parade rest.