When a file is ripped onto your PC its encoded into a format which saves the data in an abbreveated state. Your PC can read the abreveated file and decode it back into audio.
A CD player is "dumb" can only read the raw audio track for playback.
If you select 80Mb of MP3 and try to burn it as an audo CD, your PC will decode the files back into the raw audio format used on CD origonaly which takes more space on the CD than the music file on your PC