I notice that backup size is increasing at about 2MB per day with a couple of hours per day usage. Minimal repo's and apps installed and nothing uploaded.
Where should look to clear out space without affecting the install in any way.
Backup size increase because more thumbnails are being saved. You can delete every files and folders inside the thumbnails folder and it will give you a lot of space back but your system will save these thumbnails again eventually. Folder is located there; storage\.kodi\userdata\Thumbnails