I think it's the other way around, he's that way because Sargeras tortured and drained him, making him to weak to assume full sized titan form. Which is also why we're able to fight him at something exceeding getting squished and ignited with magnifying glasses.
Don't forget the new spoiler from 7.3.2. Sargeras did that from the Seat of the Pantheon, that should give you an idea of his size.