To be fair, Alvis' looks not entirely like it's part of him to me. Plus it looks a little small compared to the others. I very much think he's who we think he is, but let's not jump the gun.
Seriously though, do you think we'll see him interact with his sisters in the post game?
Also, the "core crystal" size could be because it's more a symbolic representation than a physical thing. After all, we dont know how a space time transition event of an aegis core into another dimension might change its appearance. The same rules may not apply. Plus Aegises dont play by real grounded rules anyways. I will say the zohar symbol as part of Alvis's appearance really strongly implies what many fans have speculated. Certainly not something that proves without a doubt that speculation, but it removes a whole lot of doubt from a theory that was already strongly speculated to be true before the alteration of Alvis's design appeared in this latest trailer.