I'm guessing your NAS uses NTFS.
If you are backing up to an NTFS-formatted device, you'll encounter problems with things like symbolic links. The NTFS filesystem has no understanding of such Unix-y primitives. As a result, the symlinks must be followed and the linked files duplicated on the backup.
You can get around this problem with a different strategy than rsync. You would need to create a "tarball" of the files and write that to the NTFS device. Something like this:
The file mylinux.tar.bz2 is compressed with the BZip2 algorithm. The "-p" switch tells tar to preserve all the file attributes like modification date, owner, and permissions. It also copies symlinks as links. The three excluded entries are transitory mount points that are recreated at every boot. This assumes you have mounted the NAS to /path/to/your/NAS/, e.g, /media/NAS, and are storing the tarball in the "backups" folder on that device.
sudo tar -cjpf /path/to/your/NAS/backups/mylinux.tar.bz2 . --exclude=proc/ --exclude=sys/ --exclude=run/
If you can backup the entire NAS, and intend for it only to be used by Linux machines, then you could reformat the drive as ext4 and restore the backed-up files. Then you can use rsync.