What are ISO files?
The .iso file is a digital copy of CD contents made this way: the ripper searches for the sectors of the CD that have been used, say 251,000 for instance (there are 330,000 sectors on a 74 min CD and 360,000 sectors on an 80 min CD). Each sector is copied on the .ISO file, one by one, and only 2048 bytes for each sector (only the ones containing the user data) are copied. The .ISO file should then be of size 251,000 x 2048 = 514,408,000 bytes. (It will be slightly bigger if the extractor puts a header on the file, like Nero .NRG files that are .ISO files plus a small file header).