Check you sync settings. Maybe you are really syncing items:
Do you have the right boxes checked to sync?
iTunes: Syncing media content to iOS devices and iPod
Try syncing using the manual method
Managing content manually on iPhone, iPad, and iPod
Next try:
Connect the iPod to your computer and look at the storage used by the various categories as shown in the colored bargarph in iTunes.
Come back with the results.
An "other" larger than about 1 1/2 GB indicates corrupted files.
What is the Other on my iPhone and How to Remove It
What is "Other" and What Can I Do About It?
Next, usually restoring from backup eliminated the corrupted files. However, sometimes restoring to factory settings/new iPod is required.
To restore from backup see:
iOS: How to back up
To restore to factory settings/new iPod see:
iTunes: Restoring iOS software
If still problem restore from backup. See:
Restore your device from an iCloud or iTunes backup - Apple Support
Last, restore to factory settings/new iOS device.