I've actually never seen a starfish do that. I'm sure someone will be able to give some answers. Maybe @ngoodermuth might know. Does it seem inflated or is it that he is just arching his legs forcing the body up? Maybe he ate something too big for him lol!
Hey, I have a cc star that I just recently added to my tank I fed him last night and this morning the ends of his legs had curled inward a bit, though he's still sticking to the glass his mid is extreamly bloated like yours. Everywhere says its legs curl due to disease or starvation. Ik he's not starved as I had just fed him a good meal. Maybe it's eggs? But I wonder if stars can over eat and this could cause bloating?