Both CD-ROMs and tar files use sector blocks.

For tar files, the block size is 512 bytes.

The block size on a CD is a somewhat over 2300 bytes. For data, the surplus is used for error detection and correction, so the block size for actual data is 2048 bytes. For details, take a look here or here.

Thus, in summary, for both CD-ROM and for tar files, to get the actual size on disk disk, you need to apply the same calculations, but with different sector sizes than those that apply to the harddisk.