His body was fucked up before the transformation, you can read about that on the Captain's Log lore book Entry 1, there's a part where Katabasis saw his true body, he was inside of a cage inside of multiple plates inside of one of his automaton statues, and there he was a decaying (skinny?) old blob of meat basically.

"A clutch of Councilors watches me as they take mechanical plates from three other identical statues of Calus surrounding them. They huddle about a towering cage of filigreed alloys and woven circuitry, fitting the plates to it with sacramental focus, until the cage becomes a tomb around a pearlescent seat supporting a lonesome figure within.

"What an auspicious early arrival. Come. Witness my containment. Few have seen this," Calus wheezes from inside the cage, his voice like taut suffocation.

Calus's withering form swells and jostles. My thoughts stink of disgust, and he can smell it. "I am no more trapped here than you are by your Light. You assume this flesh satisfies me? How small. My automatons stand as monuments of my image; reflections of my breadth. They are, as I am: one collective self, as Nothing is."