Cite from The Document Foundation Wiki in order to completely close problem
By defaut the documents created with libreoffice are compressed,
though, which is good since it keeps them at a reasonable size on the
local hard disk. Actually the compression is a nice feature since if
you change the ending of an an open document file to .jar you get an
archieve containing all images and other objects included in the file
in their original format.
The drawback is that if you change just one word in a document chances
are high that the compressed version of the document changes
completely - which leaves the version control system with a huge
amount of data.
But don't despair:
Libreoffice contains the feature to save documents in a flat XML
format instead: If a document is saved als .fodt file it keeps the
same data the .odt file would contain. Only that this time the data is
represented as human-readable text (which makes the work much easier
for the version control system) and not compressed.