Windows iTunes: Where are the lyrics saved to?
It is entirely possible that you have iTunes set to copy files to its own media folder, in which case the original audio files are not changed.

To find the location of the audio file iTunes is using, look under the "Where:" label in the "Summary" tab.

iTunes (both Windows and Mac) does use the lyrics field in MP3s to store lyrics, and will cause the modification date and time, as well as the size, to change.