I can't get into the technical side of it too much, but basically just all the portions of samples that fall below 0db and above the highest peak point. In other words, the parts that basically have no audio data there. So the dynamic range also plays a big factor. Anwayy, one example for instance, if you take a brand new album that has volume-compression and is maxed out for levels and then you encode it to flac, the space you'll save will be average. On the other hand, if you take a home recording in wav or something you did yourself, you might save as much as 50% of the original filesize. If it's a recording of a very low dB level, you might save even more than half.
Edit: And actually, I think I misread your question a little bit, but maybe it at least answers it partially.
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