There's a storage problem with the iPhone. You've probably encountered it yourself when trying to store hundreds of photos, apps and songs on there. Or, indeed, anytime Apple updates its iOS operating system and your iPhone tells you there simply isn't enough room.
What most people don't know is that there is a way to fit much more music on your phone, and to cut down the amount of space your music takes.
Look at the music in your iTunes library. You'll probably find that your albums are all different file sizes. Right-click on a song, and head to the "File" tab.
In the "File" tab, you'll see the file size of the song. The "bit rate" tells you how much space is taken up per second of the song. This song is 320 kbps, the maximum bitrate for iTunes.
You'll probably find several different bit rates in your iTunes library. iTunes purchases are 256 kbps, while CD imports can be up to 320 kbps.