iPhone won't back up via iTunes "Not enough disk space" But I have space enough

Hi

So I am trying to back up my iPhone 7 via iTunes onto my MacBook HDD.

I keep getting a message saying "iTunes can not back up the iPhone 7 because not enough free space is available on this computer."

My iPhone 7 is 128g

I have 162g of free space on my MacBook.

So why won't it back up?