different backup file size between sqlserver and c#

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C#

SQL

C#5.0

hi i use this statement for create backup from database in c# :

string query2 = @"BACKUP DATABASE [dbzolalmehr] TO  DISK = N'C:\Zolal DB Backups\dbzolalmehr-"+backup_date+".bak' WITH NOFORMAT, NOINIT,  NAME = N'dbzolalmehr-Full Database Backup', SKIP, NOREWIND, NOUNLOAD,  STATS = 10 ";

when this statement execute backup file size is 3 Megabyte
However
when i do backup in sql server by visual tools backup file size is 9 Megabyte

first Why ?
second , is my backup files from c# complete and Reliable?
please help..