tothergnome :
It's the way the disks are manufactured. Blue Ray files take a lot of space.
I'm not sure there is a such thing as blu-ray files. Blu-rays are just storage media the same way a flash drive is. You can store any type of file you want on it. Also, the disc is only used to install the files to the hard drive. After, installation the disc is never used again except to verify that you haven't given the game to a friend or traded it in.
It's not all that different to the way the disc based PC version would be handled. The main difference is that there is no CD key to tie to your Xbox Account so you are required to put the disc in every time you play.
According to Eurogamer Fallout 4 is 26.28GB on Fallout 4, which is almost identical to what the OP said the PC version used.
http://www.eurogamer.net/articles/2015-11-10-fallout-4-takes-a-while-to-install-on-ps4-and-xbox-one
This is probably just a case of Bethesda overestimating the amount of required space on the box of the PS4 box. The same thing happens for recommended PC specs all the time.