The Titan of Gran=Pulse in Final Fantasy XIII is bigger than a mountain as well. He eats an Adamantoise when you arrive at Pulse - and Adamantoises are way bigger than your characters, but he just grabs it and it looks like he is holding an action figure in his hands.
Have you played Final Fantasy XIII? Is the monster in the screenshot that I posted an Adamantoise? If so, that certainly sounds like an impressive battle and a contender for the Largest Boss in the Final Fantasy series.