Its hard to compare a snapsize with a filesystem without knowing all details.
What should happen.
If you transfer a filesystem without snaps, compress or dedup from one server
to another with same disks and pool layout, the size of source and newly created
target filesystem should be quite the same.
If some is different, result is different.
If you are in doubt that everything is transferred, create a filelist on both and compare.
But usually a zfs send that ends without an error is very trusty.
Is there a log where I can check if there was an error?
Is that readonly on by default for Napp-it Async? If not, maybe that's the cause of the error.
Can I send a new separate copy without deleting the existing one on the backup server? Meaning, not an incremental backup but another full backup.