Macook pro hard drive keeps filling up automatically

Same situation with my MacBook Pro 2016 with 8GB Memory and a Solid State 500GB hard drive, only it just started happening . . I deleted 5-6 GB from my hard drive and after a couple of hours I have less than 200MB free. ALSO, it won't let me do a Time Machine back up - I get message saying that my MacBook Pro's hard drive is too full and cannot be backed up. . . . I suspect it is a problem with my large Photos file keeping too many full size images, even though I am using iCloud to store the full images and only thumbnails are supposed to be on my computer . . also, a few days before the filling hard drive, Photos on my computer stopped syncing with iCloud, even though my iPad and iPhone still sync perfectly. HELP!!

You need to always keep at least 20GB of free storage space available at all times for proper operation of macOS. Running out of free storage space on an APFS file system may prevent you from deleting any files and possibly have lots of issues even to manually mount the volume in read-only mode to access the data if necessary while booted from external boot media. If you run out of free space with an HFS+ volume, then you can actually lose and/or corrupt the data on the volume.

Make sure the Trash has been emptied and that all backups have been transferred to the backup drive. If the backups have not all been transferred to the backup drive, then even if you delete lots of data you may not free up any space since the data you deleted may still remain in the pending backup snapshots.

You can use OmniDiskSweeper to look for where the largest files/folders are located which may help you to identify the issue.

To look for possible software issues you can run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.

You can try rebooting the computer which can sometimes free up storage space. Booting into Safe Mode will also delete some temp & cache files which may help to free up some space as well. Reboot normally after booting into Safe Mode.

You can use a USB3 drive or Thunderbolt external drive for storing your larger files. Or you can replace the original Apple SSD with a larger third party SSD upgrade. OWC makes an SSD that fits the Apple proprietary SSD connector, but you must be using macOS 10.13+ to use a third party SSD. The OWC Aura SSD is also faster than the original Apple SATA-based PCIe SSD.

You should always have frequent and regular backups of your computer as well as any external media which contains important & unique data. FYI, it is impossible to recover accidentally deleted data from an SSD after the Trash has been emptied. Plus an SSD can fail at any time without any warning signs even with a new SSD. With 2016+ Macs it is even more important since the SSDs are integrated (aka soldered) to the Logic Board and even an expensive professional data recovery service may not be able to retrieve any data from the SSDs of 2016+ Macs if the Logic Board fails.