Song file size in the iTunes application is smaller than it is in the iTunes Music folder.

For instance, E.S.P. has a 55.7 megabyte file size in the iTunes library, but when I look it up in the iTunes Music folder, it is larger, at 58.4 megabytes. I've noticed an increase in audio quality when pulling up the file through the folder instead of the iTunes application itself, when using third-party software such as Decibel and Amarra. Why does iTunes do this?

Note: Although it says "01 E.S.P. 3.wav", it is the only copy of the song on my hard drive. I've deleted the others.

Thanks,