2 GB Data on a CD-ROM?

I’ve recently got 2 CD-ROMs in my hand, both contain over 1.5 GB of data. One of them was Knoppix (Something like that) Linux ISO, which contains over 1700 MB. In the documentation, it is said to be done by Transparent Decompression method. Both CDs show size of 700 MB when inserted in a drive and so is their ISO image size. How do we do it?