Storage full after backup to a Mac

Hi, so I recently tried to back up my iPhone (14, iOS 17.7) to my iMac (2017, 13.7.4) and it failed. I had about 6GB free before the backup and almost none after it started and failed. In Finder, it says that those six GB are still free, and only on the phone does it fill up during the backup. Also no error message. It all seems to go into the "other" category. Is there something I can do to just make a backup of my phone or do I have to free up more storage beforehand? And also why and where do these SIX gigabytes come from?!