Why Is Eren's Titan Bones?

Eren has the same bony structure, but because his head was severed at the neck, his freaky Titan spine grows uncontrollably before connecting his torso and noggin, coming to form almost Eren's entire Founding Titan.

Why Eren's founding Titan is skeleton?

Namely, when the Founding Titan manifested itself, Eren had already been decapitated at the time. The Founding Titan's spine then had to connect Eren's head to his body, which is why the spine actually formed the basis of the Titan's body, thereby creating a monstrosity composed mostly out of bones.

Why was Eren's Titan so big?

The defining feature of Eren's Founding Titan is a huge, snaking spine. This look is because Eren was decapitated by Gabi when he transformed, which caused the spine to grow abnormally so it could reconnect his body.