Difference in space used When reviewing the space used by datafiles in SQL Server, there is a large difference betweens what SQL reports and Windows.

How can this be explained?
No different.

Space Reserved for your MDF file: 100.21 GB, which is equal to 105,082,944 KB (105,082,944 / 1024 / 1024 = 100.21)

Space Reserved for *_1.ndf: 148.62 GB, which is equal to 155,843,968 KB (155,843,968 / 1024 / 1024 = 148.62)

The numbers match up just fine.  The Operation System is going to see Space Reserved, and that's how you would need to make your comparison.

Space used is going to be how much space is used within the database file, but Windows doesn't know or care about that.  The allocation and space reserved is the significant part reported by SQL Server which is what represents the footprint on the file system.