I am trying to backup my iPhone 11 (64GB, capacity only around 60GB) on my Macbook. My iPhone has about 20GB free space, so about 40GB is used. My Macbook has 60GB free space, but when I backup through iTunes it keeps saying that my mac doesn't have enough space. Does the size of the backup correspond to the amount of space my iPhone 11 has used?
The amount of space you use on your iphone is proportional to the size of the backup. But often the receiving device will need more space than the actual backup to backup a device. Do you have Google Drive or Dropbox? If so you can move some of your files off your MacBook and store then on one of those services. This will free up some space on your MacBook.