Not enough space on iPhone to restore.

Why is there not enough space on my iphone to restore from a backup? I have a 16gb Iphone 6s Plus. It has 3 GB of free storage. Itunes says I cannot restore because there is not ample free storage on iphone. Why do you need more than 3GB of free space, which is literally 25% of free space available on my phone, to restore?

Are you making copies of my files in the restore process? What is the point of this? A restore is literally a restore. If I have to free up to 6gb of space, that is literally 50% of my available storage capacity. That's kind of absurd just to RESTORE from a backup.