Backup and Sync Overhead ?

I have been using WSA for a couple of years now but have only recently explored Backup and Sync.

I started with Sync and one 100 MB file. After a couple of hours WSA had uploaded almost 500 MB. In other words, the same file four or five times. I turned it off. I have slow internet and hard data limits and cannot afford that kind of overhead. So Sync won't work for me because I can't control when it will upload. My data limits averages 320 MB per day peak and 320 MB per day off peak.

Then I went to Backup instead of Sync and trigger the backups manually in the early morning ( off-peak data ). Even so, Backup uploads the file two or three times, meaning 200 MB or 300 MB for a 100 MB file. I watch the "file transfer" window so I can see that it is uploading the same file two or three times. Why is this? Why so much overhead?