As far as I remember he was never called short in the books, right?

Two things come to mind with that-- for one, there’s sort of a trope of the ‘little old guy who’s paradoxically, extremely dangerous.’ (and Gurney was the war-master, right?) Yoda and Gollum come to mind as pop examples, but the core of that might go back to folkloric, mythic examples, such as Rumpelstiltskin, fey faeries, elves and so forth. But I’ve also seen that kind of thing before in various modern lit.

…And I kind of like it, frankly! You know, as opposed to the more common ‘look at me, I’m muscular and badass-looking’ character, which is kinda what Marvel/DC have flogged to death over many decades.

The other idea is that concept art is notorious for being experimental, and of course, intentionally so. So an artist might create half-a-dozen odd character designs to show the production-designer / director, letting the boss pick and choose elements they like best to create the final physical version of that production’s character. All very SOP and common in these cases. So you might even find a concept drawing in which Duncan is the shrimp, and Gurney, the big, imposing guy. :D

Eh, pardon the ramble. It just sort of popped out, haha.