Quote Originally Posted by Pete for President View Post
Who says Sin looks the same every time it gets reborn? For all we know it could have been something totally different the first few times around. SpoilerLike a unicorn! After all, the Final Aeons that defeat it are different too, and Sin is built around them.

Speculation aside, the fact that Sin resides mostly in the ocean makes sense for it to be a whale. On the other hand, at some point it grows wings and has some sort of legs so out goes the theory.
That brings up an interesting point -- do we think Sin looks like a whale because of Jecht's ability at Blitzball, a primarily underwater sport? Maybe Jecht's characteristics are built into the physical manifestation of Sin. (SPOILER) Although this does seem implied when Jecht talks about how he's losing himself more and more as time goes on