I don't know whether it's discernable, but on the head, he also has a bionic eye, and there's the antenna fitted too (which can only be seen as a sort of rod jutting out in those pictures.
As you say, any intended 'improvements' of the imperium to a tyranid bioweapon like a gaunt would only serve to worsen it, and with Victoria 's fetishising of the intrinsic strength of the tyranids, I imagine that she would instruct that the Magos Biologis only make those adjustments which are necessary for control of the beast.
As to the variance in build, my thoughts were that the Magos would be outfitting the Gaunt to maximise its lifespan, so through a program of selective breeding, chemical and technical enhancement they would be making their gaunts somewhat more hearty than they are usually. In fact, I purposefully posed this figure to be in a static, intimidating pose rather than a moving one to convey the fact that since being 'servitorised' it is no longer the agile predator it once was (with a chain added for the obvious reminder that he has been made into a slave) and is now little more than a material display of Inquisitorial muscle.
Still, I definitely appreciate what you're saying - seeing as Warriors come in a pack of 3, i'm still debating whether to turn the remaining two into variant Gaunt-servitor models or regular tyranids as GM Nasties, so i will bear your comments about relative build in mind for those.
Thanks, and any further input is appreciated and welcomed.