Disk space requirements for MS-DOS
If you’re using a disk compression utility, then the actual amount of disk
space available may be less than reported. This is because the compression
utilities usually report the amount of free space available assuming that the data
which would go into it would be compressed at least as well as the data already on
the disk. This is important information if you’re trying to install the tools
onto a compressed disk with only just enough room for the installation. The
installation might not complete because the compression utility can’t do as good a
job as expected.