MacBook Storage Management

So I unfortunately have a MacBook with only 128GB of storage. I had 6GB left of free space and in an effort to free up space I moved the photo library which was taking up 14GB (due to being low res iCloud Photos previews) to an external drive. 6GB + 14GB = 20GB of new free space one would think. However after 2 days my free space is only 10GB? So now even if I wanted to transfer the photos library back to my Mac I won't be able to so I really don't understand how the storage is managed. I know the ultimate fix would be to upgrade to a new(er) Mac with a bigger storage but I'd like to wait until this one (2017 MBP) doesn't receive any more updates