SQL backup file size reduced
Check to see the amount of space actually used in your database. You probably have a lot of free space available. A quick way to do this would be to connect to your database and use exec sp_spaceused which will give you the database size and unallocated (unused) space.

When SQL Server backs up the database it only needs to backup the pages that contain data (and the log), which means that the backup can easily be smaller than the files themselves.

This question has also been answered in Why is a .bak so much smaller than the database it&#39;s a backup of?