I think Kopia should deal fine with this, though I don't think I have many such large files that I modify. The reason why I think it's fine is that it doesn't

I think Kopia should deal fine with this, though I don't think I have many such large files that I modify. The reason why I think it's fine is that it doesn't really try to do "delta diffs" at all. Instead is uses a rolling hash to cut the input file into variable length chunks and those chunks are added to the chunk storage, and then actual files are constructed from those chunks. Obviously such copies should be done when the files are stable, so either not running or using a snapshot. (If using LVM, this can be arranged easily with any filesystem, except ones that don't like getting mounted multiple times..) So while I guess size-related bugs could be there, there's no any fundamental property of making the task bigger as the files themselves grow bigger.