Hello,
How are you moving the files to the drive? Are you doing that manually or using the WD Smartware backup software?
Make sure that you are not doing both.
Please note that the size of a thumbnail should be around 1 or 2 kb so, it wont use that much space, it might be something else taking the space of the drive.
If you are using the WD Smartware, please check how many version of the files the unit is keeping.
