bad archive of changed files

you made "tar" files inside the archive.
in this archive many similar files, many duplicated information. in this case you should use solid archiving.
but archivers(packers) use only little dictionary. so when one "tar" file is smaller then dictionary, when the beginning of next tar file goes into dictionary the ending of the first in it. so packer does see no duplicated information.
so if you make folders instead of tar files it is ok. in this case it is so. because solid archiver places files with equal extension and then with similar names (by alphabet) together. in this case they are very similar. (maybe, example is: viewtopic.php in 3.0rc8 to 3.01 and viewtopic.php in 3.0rc7 to 3.01)
i tested it. your phpBB-3.0.1-files.tar.bz2 is 2843099.
i made 0phpBB-3.0.1-files.tar.bz2.7z 1742956 and
0phpBB-3.0.1-files.tar.bz2.rar 822513. both solid, maximum, 7z is ppmd. (don't give attention to "tar.bz2" in file name, no tar nor bz2 in it)