I’m doing a backup of .eml files to Dropbox. The local source is about 19Gb, but the backup in Dropbox is only 9Gb, or half the size.
I have other backup sets of “normal” files, in which the size difference is about 20%.
I know about block backup efficiency, but is such a big difference (half the size) for .eml files normal?
I suppose it is very much possible, but depends on what’s inside the EML files.
If there is a significant number of EML files that contain the same attachments (forwarded messages with for example some pictures attached), deduplication may occur, resulting in just one uploaded version for all EML files containing the same data.
The difference in size could also be contributed to (possibly even more so than deduplication) by compression. Since EML files will likely be mostly text, and text compresses really well, that may be a big part of it.
If you feel like testing (and have enough free disk space), you could try a test compression of your local source with the same parameters that are being used by Duplicati and see how big the resulting file is…
I’m not sure about that, but 19 GB of text seems very, very huge to me. I expect that most space from that 19 GB is occupied by attachments.
Good point - I wasn’t keeping the absolute numbers in mind.
It would indeed make more sense that TowerBR has simply been getting a bunch of emails from my mother each asking “did you get these pictures yet?” and containing the same 20-30 full sized raw images in each email. 
Most likely. I really don’t have a statistic on this, it’s all old emails that have to be saved for archiving reasons. They are not active accounts.
Maybe not from your mother 
, but the staff here exchanged a lot of attachments.
Ah - lots of attachments and files for more than one user, I’d say deduplication for the win. 