That is actually a limitation of Windows itself. As Kopia mounts your repository through WebDAV, the file size limit of Windows WebClient service comes into effect. As you noticed, that is 50 MB by default. Take a look at here for a possible solution: https://kopia.discourse.group/t/error-0x800700df-the-file-sizeexceeds-the-limit-allowed-and-cannot-be-saved/951.