ok, thank you for the information, and for the hint to bc classic!
Indeed my current setup is to *sync* all (changed) files to the bc-folder -> they will be re-encrypted and thus uploaded again completely, as I've learned now.
I might change my setup so that the original files reside within the bc-folder by sharing the mounted bc-volume instead of the original location - I wanted to switch over to that setup anway, because I don't need the sync process then (just wanted to run some tests first). BUT: For the database this is no option, because it runs inside a virtual machine, and I don't want to move (and thus encrypt and upload) the whole VM-file. Currently I run a script inside the VM copying the database-backup-file to a subfolder which is backed up like all the other files. But even if only some blocks of the 1.5 GB file have changed and I use the same name, bc 2.0 will re-encrypt the whole file -> dropbox will upload it completely. Right? So let's have a closer look at bc classic...