Codecs

[sphinx3212000]

This may not be the appropriate place to ask this question but oh well.

I was wondering if someone could explain the difference between the decoding side of a codec with what happens when a file is unzipped.

I know of course that when a file is unzipped it is returned to its original larger size but how does this decompression compare to the "decompression" by a codec since mp3's,video files, etc remain in a compressed form on your hard drive.

Does the decompression only occur as a process in realtime during playback while the actual file you downloaded remains the same size?

Thanks