I recently started testing Duplicati with a mult-GB backup and have several of these file size errors on my system while backing up to a NAS file share. I looked through the source code and found that the FileBackend module uses a simple stream copy with no verification at all. For such an important operation, this seems like a huge oversight by the developer.
I was able to reproduce the issue by unplugging/plugging the network cable during backup. Sometimes this does not cause an issue, sometimes the file size is different, and sometimes the files does not get created at all. There are likely many ways this issue could occur but proving that something as simple as a network interruption can corrupt a backup set is unacceptable.
I am working on adding file size validation to the FileBackend module code that would detect and retry the file creation if the file is missing or the wrong size.