Pretty sure that's not the case. I have 898 MB of photos in my Photo Stream, and only 35 MB in my camera roll. Yet I still have 3.2 gigs of messages? That doesn't make sense. I'm assuming videos are somehow taking up that space, and I can't find those.
Have you ever restored your phone as new? I want to say this is an old bug that's been fixed but still would not have cleared the old cache, but I could be wrong about that. Any restore as new since then would prob clear that old cache.