[justme71]

I have nearly 3100 songs in my iTunes library, that is listed at 27.7GBs. When I sync them onto my 32GB iPod touch 4 it takes up only 12.4 GBs.
When I sync them with my 64GB iPhone 4s it takes up 27.7 GBs.

Why is it so different?

[phessler]

Do you have the setting checked off in iTunes for "Convert higher bit rate songs to 128 kbs AAC"?

This would make that difference.

[Kaitokomauri]

It might also be something rather simple as a difference in the individual device sync settings in iTunes.

Check whether the same options are ticked under the 'Music" tab in iTunes from each device's settings. I.e. "all music" or the same selected playlists under "Sync music" are ticked for both your iPod Touch and iPhone.