This comes down to the difference in a binary numbering system and decimal numbering system.

Simply put 1MB is not actually 1,000,000 bytes

• One Kilobyte is 1024 bytes, and one Megabyte is 1024 kilobytes
• Therefore one Megabyte is 1024 x 1024 bytes = 1,048,576 bytes
• If you then store 700MB on a CD, you are actually storing 700 x 1,048,576 bytes, which equals 734,003,200 bytes

A little bit is generally used for storing the locations of the info and trying to make the discs as compatible as possible so you end up with around 730MB's.