How to stop having iOS files on my macbook?

1. I have already relocated my itunes back up on my external drive.

2. I have already deleted my old itunes back up files on my macbook. That's why i got a lot of free space.

3. However upon my next back up, i checked my macbook storage and to my surprise, it still consumed 45gb on my macbook. Although external drive is the new back up destination. How do i stop this? I want all my iOS files on my external drive.