Have you ever bought a hard drive, and then wondered why it wasn't the size you thought it should be? For example, consider a 320GB Western Digital hard drive. After you install it and format it, you only have 298GB of space.
22GB went to the NTFS file system. (This assumes you have Windows XP or newer, and formatted it with the suggested sizes.)
CD is the same as a hard drive -- it's just another storage medium. Computer data needs a file system. And that file system takes up space.
CD-Audio is modulated digital information that has no included file system. There are no files on an audio CD.
When you "rip" a CD, you put the data into a file container, which adds some overhead. And then it's being housed in the file system, and you see it's size as reflected by that point of reference.
There are other ways to burn CD-R that allow for more data on a disc. That file system skips some of the error correction built into the standard CD specs, and therefore is a more unreliable disc. (Compare it to your stomach. It currently exists at a certain size. You can choose to put more in there, but you'll have to suffer the consequences!)
Remember how I wrote "formatted it with the suggested sizes" up above? Hard drives can be altered to different sizes depending on how you format them. Different file systems can also change the amount of space available for data. Linux, Windows and Mac all have native file systems, as do other operating systems. The XBOX is a good example, too. It uses a modified version of FAT32 (FATX) so that you can't stick it into a computer and read/write data. Windows sees a blank unformatted drive. Technically Windows sees a blank CD Audio, too, but it puts track links into Explorer view, which instigates a media player when the files are run. (Notice that you can't copy the "files" from a CD Audio to your hard drive. Ripping is a required process. There's nothing actually on the disc -- it's just what Windows is showing you.)
I know that probably breaks your current understanding of how discs store data. Admittedly, this can be complicated to wrap your head around at first.
Hope that helps.
