Why was the folder size unmatched to the size on disk?

For details, I have two (2) environments, one for the production server and one for the backup/DR server. Upon checking, the copy files from the production server were unmatched to the DR, and here's the example info:

Prod:

• Folder size = 500 GB
• Size on disk = 2.0 GB

DR:

• Folder size = 500 GB
• Size on disk = 501 GB