In lore regular trolls can very much stand up straight, the darkspear mainly hunch to not tower over the other Horde races as a show of respect. While the zandalari stand with a straight back due to their pride and showcasing their “elegance”. No reason zandalari can’t be hunched and “skinny” either, most notable example is Zul.
And the differences between the different troll races in appearance is mainly small height differences, skin tones and other such small details, about the same level of differences as between DID, bronzebeard dwarf and wildhammer dwarves. And I don’t think it would be insane if those three dwarf races was in a single race with a bunch of customization options to mix and match.
Edit: adding some pictures showing the trolls so it’s easier to see how alike they are. The zandalari are not that much bulky compared to the darkspear, it’s on the same level as the dracthyr options. The second picture is of a zandalari using a regular troll texture with no editing of it, showing how well it already fit without any adaption, only problems are really the tusks not showing and the jewelry.
Also posting a picture of Zul and Zul’jin to show trolls not having the “standard posture” of their so called race.
