Final Fantasy XV: 10 Perfect Fan Theories That Make Everything Better
5. Astrals Inhabit Messengers...even After Death
When Noctis summons the Astrals, we see their true size and power
– yet there are countless references to the Astrals communicating
with humans. This is represented in the cosmology books, but also the
accounts of Ardyn, as well as Noctis' interactions with Gentiana
(Shiva) and the messenger dogs, Pryna and Umbra.
Of all the Astrals, the only two that appear slightly close in size to humans are Shiva and Ifrit. Shiva we know was once as large as the others, until she was killed at the battle of Gralea, forever immortalised as a tower of stone and turning the desert into an icy wasteland.
However, she lives on in the events of FFXV, taking control of her former messenger, Gentiana.
This would explain why Ifrit also lives on. After his battle against the Astrals, he inhabited his messenger (the form we battle against at Insomnia), returned to The Rock of Ravatogh to reclaim the crystal (the heart of Eos), and went on to build his story at the Pitioss Ruins.
It also explains why the body of the god of fire is covered in scorch marks – it's a mortal body.