Alternatively, if somehow the changed parts in the 5MB file sound up scattered in 10 different dblocks, you may download 10GB to piece it together. 1GB is a pretty large increase from the default 50MB, although if your network won’t download a 1GB file, I’d have expected you’d be seeing that in the sampled verification done after every backup (unless you had also set –no-backend-verification or –backend-test-samples=0).
Choosing sizes in Duplicati has some notes on sizes. You can change –dblock-size whenever you like. but there won’t be repack just then. Over time, backups, deletes (per retention policy), and compacts will do it.
I’m not sure where this issue is now. It sounds like the main objective of the restore was achieved on a new attempt for which we’ve seen no info. If you want that pursued, –log-file portions at –log-file-log-level=Retry would give a better view of what’s going on, including a lower-level (but not lowest-possible) backend view.