The figures quoted vary and are given as an estimate really.
the size of the file will vary depending on the length of the song (longer song = more information = larger file) and also the way the song has been encoded (there are lots of different file formats, each with their own benefits) and also the bitrate the file has (this is a setting that it made when the song is ripped from its source (like when you put the cd in to the computer and iTunes (for example) imports the song from the CD, a higher bitrate keeps more of the song information but the cost of that is that it makes the end file larger.
all of these factors go in to processing of the file and changing these settings will result in a smaller or larger file depending on the options you choose.
edit: beaten to the answer while i was typing my post up