Did you know a single backup file can contain multiple database backups? When you backup a database to a file, if that file already exists, then by default the backup will be appended to the existing file. Causing the file to increase in size. If it’s the same database (and yes you can have a single file containing backups from multiple databases) then the file size will double or more. This behavior is controlled by the INIT/NOINIT clause of the BACKUP DATABASE command. NOINIT (the default) tells SQL to append the new backup to the existing file. INIT tells SQL to overwrite the existing backup files. Note the header of the file is not initialized.
This is good information to know.