Tape consume extra space then actual upload size

We were taking Tape backup (LTO) from Windows server folder.

After backup was completed, when we compared the size on the Tape was different from the actual size on the windows folder.
We could not able to find where the extra space consumed on the tape.
Windows Folder size: 58 GB
After Tape Back, size on tape is: 90 GB
We could not able to identify this extra 32 GB

How and where on tape drive we can identify extra utilized space/files/waste ?
and how we can remove it ?

Thanks&Regards

Sachin