Magical change of appearance in the lore

[shadowclasper]

Or Yojimbo being "skinny Gilgamesh" and somehow becomes fat again when he decides to reveal himself.

Okay, so while I can agree with everything else you've said. This one actually does have an entirely canon explanation.

Glamours.

Glamours are straight up stated to be full transformations of appearance. It's why you can wear say, the chocobo head or literally any big and expansive armor set and then have it glamoured to 'the emperor's new X' or something else, and vice versa.