Incorrect size reported by MyCloudMirror

I use MyCloudMirror 2TB for backup. My Windows Desktop had about 280GB of data to backup. This is about 30G of videos. However, after backup, the dashboard for MyCloudMirror reports that it has used 340 GB of diskspace. Also that I have 100+GB of videos. Does anyone know why this discrepancy? I tried doing spot searches, and did not find any extra files etc. Is there a way to reset these counts?
Also, if I want to delete the existing backup and restart everything, what is the best way?

If you are using the WD Smartware backup software, you might have a setting to save multiple versions of the same files.

Check the settings tab and see how many version of the files are you keeping.