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ever since the arrancar started showing up and releasing i think we could guess what some arrancar releases could look like now too, considering what some arrancar's releases have been like after we've seen their original hollow forms too.
if you look at the silhouette of an vastolorde, which is obviously ulquiorra, he will probably have that basic shape after he releases. the same with yammy and that adjuchas silhouette and wondwerwice when it showed him before he became an arrancar (so wonderwice should look pretty cool when released).
also this is kinda disappointing cos since vastolorde are meant to be very small, human sized, that would mean the vastolorde arrancar would have very small releases compared to other which have been shown. like just covered in armor.
this theory could make sense too cos it wouldn't really make sense for a vastolorde arrancar to have a big release considering that when an arrancar releases they return to their original form (and a vastolorde's original form will be definently small).
aroneiro's release was very large, which makes sense since he was a gillian - gillian are very large. all other arrancar who have released have not been as giant a this, but they all have been big (just not as big), which suggests all arrancar who has released so far could be adjuchas. grimmjows release has been quite small even though he is an adjuchas, which also makes sense cos he could have been close to becoming a vastolorde, which is why he could have been so small.
i also think that shawlong, ilforte and edorad (maybe d-roy too, but definately not nakim) have adjucas bodies based on things i wrote (cos they should have been adjuchas in the recent grimmjow flashback), however i also believe that they are now classed as gillian cos they said they gave up on evolving in vastolordes cos since shawlong said it was just their destiny to become adjuchas, maybe they stopped eating hollows which lowered their power level, making them as weak as gillian. that would explain why aizen and hitsuguya said they were gillians (just cos of their strength) while ishida/urhara said that grimmjows team was made up of gillian and adjuchas (they are adjuchas, just as weak of gillian).
He said that they weren't Vasto Lorde, and probably not even Adjustas, and Aizen said that they were all (sans Grimmjow) gillian.
As for how their power levels got lower, that's not made very clear, since they even say Grimmjow ate them, which as we can see by them being alive, never happened.
And as for your "small release" theory, I've brought up a similar question before: that Adjustas's releases are more humanoid than gillians.
The reason that their power levels dropped is that they gave up on becoming vastolorde, and stopped eating hollows. That would make them drop from adjucha to gillian.
There might also be a different ranking system for Arrancars and Hollows, like Gillian, Adjuchas, and Vastolorde within the Arrancar class, and the same ranking within the hollow class. So maybe they were adjuchas hollows before undergoing shinigamification and ended up in the gillian class Arrancars.
But this is highly unlikely.