If you have a lot of music in your iTunes library, you might sometimes prefer not to have to go through all of your songs to determine which ones to copy onto your iPod, iPhone or iPad, so you let iTunes automatically pick songs for you. If you don't want iTunes to automatically fill up the remaining space on your portable device, you can disable the autofill feature, and sync only the specific music you want.
Launch iTunes on your computer.
Connect your iPod, iPhone or iPad to a USB port on your computer with the device's included USB cable.
Click the icon of your device in the left pane of the iTunes window.
Click the "Music" tab at the top of the iTunes window.
Click the "Automatically Fill Remaining Space With Songs" check box to deselect it. Now iTunes will only sync your device with the music that you select.