The folder I am backing up is about 30TB and maybe has 5-10% changes per day. It is stored on a 12x 7200RPM Disk array with an ReFS formatted volume, attached to a Veeam Repository server.
My Veeam Job Summary page says it processed and transferred 152TB of data in 7 hours at 6.4GB/s and fit it all on a single 9TB tape. Sounds a bit fishy to me!

So as I said, it seems to be copying the actual changed files to tape, so it's doing the job. But I can't figure out where my crazy stats are coming from or if it's a problem. I get at least a few days a week that look like this.