I do one full duplicity backup (through Déjà-Dup) of my /home (except a couple of directories) about once a week. Now I found that I saved some very big files that I don't really need to include in my backups, so I'd like to remove them from the old ones to save some disk space. Is that possible?
No, you can only list contents, create or delete a backup.
But is it really an issue? Duplicity/deja-dup creates incremental backups so if that large file changes only the changes should be in the next backup. Not the whole file. Plus the whole backup will be encrypted and have a hash attached to it to determine if the backup was tampered with.
You could move those backups to an external disk or to an on-line service and then delete those to create some space and set up the new backup to exclude those files.