I would say your SSD is using 650GB of storage as being reported by "Get Info". The "file size column" in the Finder is not showing everything on the SSD. I believe the storage space indicated at the bottom of a Finder window when you highlight a folder on the volume should show the same thing as the "Get Info".
Make sure your backups have all completely transferred to the external media, otherwise a lot of "deleted" items will still be on the SSD within an APFS snapshot waiting to be transferred to the backup media. Not intended to be insulting, but also make sure the Trash has been emptied (can be easy to forget to check if you are concentrating on "moving" files).
Reboot the laptop.
Are you using any cloud file syncing services which may be refilling the drive?
Are there multiple user accounts which may hold data that can be moved or deleted?
You can also use OmniDiskSweeper to see where the largest files/folders are located. Data in any APFS snapshots will not show up.