syncing ipod with itunes

I recently downloaded the latest version of itunes on my pc. When I plug in my ipod to sync it, I am receiving a message that says the ipod cannot be snyced because there is not enough room on my ipod to hold all my songs in the library. My ipod has 64g with 58g available to use. Right now my ipod says I still have 11g available for use. I made sure that I checked the "sync only checked songs". Itunes says I need an additional 21.7g. HELP