Why the backup file size is much larger than the restored database I got a 18.3 GB .bak file, when restored, the database size is about 78 MB, including logs (54MB).

restore filelistonly from  DISK = N'E:\xx.bak'

BackupSizeInBytes
23068672
0


I understand that the opposite (backup smaller than restored database) might occur if there is unused space that was allocated in the source database, but this is not the case here.

Why the backup file size is much larger than the restored database?

The size on disk is 19,200,123 KB which is about 18 GB.
23068672 bytes = 22 MB, not 20 GB.

If it were really 20 GB the only thing I could think that would make it that big would be reusing the same file for multiple backups and appending each backup to it.