Did WD Backup really Backup

I’m new to this. I bought a My Passport Ultra 2TB, installed the software and ran a backup. The system took a long time to Prepare for backup. I went to dinner and came back and saw that the screen said it had backed up 903 GB. I thought, great. Then I looked at the properties of the Ultra and see that it only had about 421MB of space used and 1.99TB available. This makes me wonder if things really got backed up. Any thoughts?

It backs up select files it does not make a complete system image. It may use compression also.