You can download one of your backup files and open the zip file using a 3rd party zip application like 7zip, and check how much space each folder takes.
This will give you information on what is making the backup so big, and be able to make adjustments.
OK that was my idea also.
I did it, and 97% of the files are in the "provisioning" folder, most of them in "firmware" and "firmware_new"
It probably keeps all old fimware forever? Where can I clean this?