Something I had been remembering recently is that 'Stompy' (the nickname given to the Xenomorph Drone from Alien: Isolation were eff-off big compared to most of their brethren; Your average Xeno Drone or Warrior is about 7 or so feet tall, but Stompy and the others were around 9 or 10, being nearly as large as Praetorians! This is especially considering they came from the same 'clutch' of eggs as Big Chap who was normal sized for a Xenomorph.
Personally I have a few ideas or theories as to why they were able to get so big beyond the game devs wanting to really make the Aliens feel like unstoppable juggernauts due to scale.
1. An abundance of resources allowed them to get bigger, as if I recall correctly Big Chap's rapid growth was partially due to finding food as a Chestburster and gorging themselves, and this was on a comparatively rinky-dink ship rather than a much bigger supply depot space-station. While Xenomorphs don't NEED to eat, we do know they can; The Dragon actively fed on the people it killed, and other sources like the AVP movies, games, and Alien novels suggest they do in fact consume hosts for food. Granted, the canonicity of some of these sources is VERY tenuous, but we do know Xenomorphs can eat in some capacity, so perhaps that occurred here?
2. They're a unique 'phenotype' or evolution; As much as I'm not incredibly fond of any single Xenomorph/Xenomorphs being able to 'moult' into a Queen willy-nilly, it could be possible that Stompy and the others were developing into a more 'Royal' caste (digitigrade/spurred legs*, long-bladed tails, increased stature and intelligence) whether they or the others were becoming Praetorians and ultimately Queens, or perhaps some sort of other phenotype that allowed them to breed rapidly (ie sort of an 'Eggmorpher' or 'Drone' in the sense of them being 'reproductives') as there hadn't been a Queen on Sevastopol during the initial 'outbreak'
Of course, this is all just my own personal stupid conjecture, and I wanna hear your guys' theories or ideas as to why these guys were made so friggin' big!
*I am aware that the actual reason is plantigrade legs wouldn't animate properly but it's still something to consider given that other Xenomorph Drones in other games/canon possess plantigrade feet and legs.
