Uncompressed audio files take up about 10 MB per minute.
When you import those in iTunes in the format of your choice, it takes up the same or less space on your HD.
Let me give an example:
Imagine a track on an audio CD with a play time of 1 minute.
When imported in iTunes, the file sizes will vary with the format as follows:
AIFF 10 MB (uncompressed)
Apple Lossles 5 MB (compressed, but lossless; retains audio quality)
MP3 1 MB (compressed, but lossy; loses some audio quality)
AAC 1 MB (compressed, but lossy; loses some audio quality, but less than MP3)
Regardless of the file sizes, playing time is still 1 minute for each of those files.
If you burn those different files on an audio CD, iTunes decompresses these files and they each will take about 10 MB on your audio CD.
That's why you can't fit more then 70 - 80 mins. on an audio CD.
The only workaround is, to burn an MP3 CD. iTunes will burn compressed files on that CD. That's why you can fit more music (more play time) on those discs (up to 8 or 9 hours).
Many recent CD- and DVD-players can read MP3 CD's. These players have decompression software on board.
Older CD-players don't have that feature and don't know how to handle these discs.
Hope this answers your questions
M
Note: the file sizes I mentioned in the example, are about true for the default settings for each file type in iTunes. I recommend not to fiddle around with those settings if you don't know what they mean.
