After a lot of effort I used my BDROM to make an image of my MGS4 BD. Its size is about 46.6 GB but all of the files inside the image together have a size of 33 GB. So what is the extra size for?
After investigating files inside the MGS4 BD,I found a suspicious file named "demo_n.dat" with the size of 0 KB but I am suspecting this file for being a big file filled with not in-game demos. If you pay attention during the game, there are some demos which seem to not be an in-game demo because of better texture quality comparing the game and also snake's default clothing!
So I think there must be something inside the BD which is normally we can not see and some how it is hidden.