– This is a misnomer. Strictly speaking, any file that has been compressed using a lossy codec (MP3 etc) can never be CD-quality.
– Ask yourself this: If a track on a CD is 47 MB and its compressed MP3 is 6.2 MB, how can they contain the same information? Unless the codec is truly lossless, they can’t.
– It’s that simple. However, files that are lightly compressed can sound quite close to the original file, but the less you compress, the bigger the file size becomes – this should make sense, because the less you remove, the more like the original the MP3 sounds, and the less you remove, the closer (in size) will be the MP3 as compared to the original.
– On the other hand, truly lossless codecs (FLAC et al) faithfully reconstruct the original file upon playback and restore its fidelity; however, the price for this fidelity is that file size compression is only about 50% as compared to some MP3 rates, which can reduce the file size by more than 90%.