Had 500GB on hard drive before installing Big Sur. After installation I had 89GB

No need to reinstall.

Just delete the extraneous Data volume.

As Barney said:

In Disk Utility, you see two Data volumes:

• one has mount point /System/Volumes/Data - that you want to KEEP.

• the other has mount point /Volumes/Macintosh HD - Data - this one is the old one and you can delete it right there.

Select the volume, control-click and choose Delete APFS Volume.

one of the Data drives.

Unless you have two OSes installed, you should only have one Data volume.

That is why I asked how you reinstalled. If you have more than one Data volume, you didn't do a very "clean" install. Clean install is a completely useless term because nobody uses it the same.

You likely still have the Catalina Data volume. You can remove it in Disk Utility.

The "in use" Data volume will be mounted at /System/Volumes/Data.

The orphaned Data volume is mounted at /Volumes.

You can see the Mount Point in the info pane in Disk Utility when you select the volume. You can Remove the volume by clicking the Remove Volume button.

Depends on what you had on it before installing and how you went about installing.

If it was an empty 500 GB drive, it should only have about 20GB used after install.