Snapshots are not files as such they are meta data. More like a diff they have only the data that has changed not data that has not so it looks larger then it is since they reference the unchanged+ the changed.
df/du utility does not understand snapshots you need to use the BTRFS and snapper utilities. DO NOT try and remove by normal means you will break the file system
have you: rolled back, ever mounted rw to other subvolume, created snapshots not using snapper? other operations?
for more information:
sudo btrfs subvolume list /
sudo btrfs qgroup show / (if you have quotas enabled)
sudo btrfs filesystem usage -h /
so i have never done a full rollback (only booted read only) but from studying manual method, it seems the procedure is rename snapshot to @, rename old root, then delete old root. A guess would be that 530 is now your root and you need to delete the old one. (please investigate before hand).
cant seem to edit my last post… i would have thought however that snapper would have marked the old root for deletion. There is a possibility that there is nothing wrong, df etc give wrong results. I shall stop rambling now…