Well FFXI, FFXIII, and FFXIV are all over 8GB, to give you some obvious examples, as well as almost all of my PS3 games that I run from HDD.
You are right about the cache though
That would really bump up the size.
Then again maybe cache isn't really needed for the field scenes, and aali could disable it for those?
Also like sl said, wouldn't the lack of transparency make jpegs pretty limited?
I don't think comparing it to lossless videos is really fair, since I imagine the file sizes would be even bigger, and for such brief sequences in the game.
But the field scenes are the primary visual in the game, totally worth it.