The phone is partitioned into many partitions, Yours has a system partition of 6.05GB. You can't "decrease" it without repartitioning storage and, unless you fully understand Linux, that can lead to a totally borked phone, so I'm not going any further in that direction.
You can see what's happening by installing by installing
Terminal Emulator for Android and typing
df
then Enter.
You'll see (but can't change) all the partitions the phone is divided into. You only get to access the last one for your apps, music, pictures, etc.