Good theory. The amount of Ether also controls the size of fallen. The amount is channeled through an Elder Cypher.
On one of the cyphers steps it says;
"Fallen barons take big draughts, grow tall. Dregs with little sips stay small"
Not sure if it's helpful or not.
English
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This evidence further supports my theory. I suspect natural Fallen are the general size of Skolas and Aksor, but only the strongest of their feudal society can now return to this size.
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If ether is what makes them grow I'd say dregs are the true natural size. They are basically similar to humans in every way aside from arm count.