> compresses them with tar and gunzip, I then with duplicity use full
> backup initially then incremental after.
This mechanism will guarantee that the incremental ends up being about
the same size as the original. Duplicity saves space by only sending
the changed parts of the file as part of the incremental. By using gzip
compression, you defeat the comparison process, so each file is totally
changed as far as duplicity is concerned, thus the large backups.
To make best use of duplicity, do the mysqldump straight to text files
and make that the input to duplicity. It will compress and tar them
before sending to the remote system and you will see much smaller