Hey Juan.. mainly you lose a little bit of space because of formatting. For example when you buy an mp4 some space is lost in formatting and also because of the firmware as firmwares take up 5-7MB depending or more. Formatting also takes away 1-2 MB. I have a few 1GB players but most of them only have 965MB to begin with and thats mainly because of formatting and firmware reasons.
Hope this answers your question!