I really like the gigaskin theory. It makes sense, it explains their love for Allagan locales, and even a "devolved" look makes sense. I have a big problem though. Looked around for some examples of Acheron:



Hes not just giant big, or filled up with Allagan roids big, he is enormous. Compare the gigas of Eorzea:


(Why is a DoH tanking that thing?)

The closest I've seen in height is this guy:



The only thing I've got to work with is the moogles (their bards in particular) are apparently privy to a magic that enlarges their team mates to well beyond normal size for a period of time, Maxi-moogle, I believe.

Maybe that's what happened to this typical Elezen woman after she made a pact with the voidsent:


(Wha.... that's a Roegadyn... what the?)