StarDeb55 is correct here. Higher bitrate = larger files. The songs in iTunes plus (bitrate of 256) are going to be larger than the same song from the "old iTunes" bitrate of 128.
A format that's more compressed will be smaller than if you imported from the CD and kept it as the "full size" file. It can also depend on the length of the songs. If you've got 5,000 songs, and many of them were longer live versions of songs--that changes things also. You could imagine that a studio version of a song that might be about 4 minutes, might now be 8 minutes in a live version--and be a much larger file size.
If you just look at a handful of the size of the songs in your library you'll have an idea of where you stand. To see this, select the song in iTunes and go under file-> get info->summary. You'll see the file size there. If you've got a bunch of songs that are about 4-5mb in size and you use 4.5mb as an estimate of your "typical song size" (not uncommon) you can see that by multiplying that by 5,000 you could easily be around 22gb of music
iBook G4; eMac 1Ghz; iMac DV+ Mac OS X (10.4.9)
