There are third party applications that allow you to search and delete redundant files.
Make sure you do not delete files from the backup folders as this might cause corruption on the backup.
The fact that the drive on the computer is smaller does not mean the external drive can not get full.
This will only depend on how much data you put on there.
For example, you can transfer files to the external drive that you want to keep and delete them from the computer since you don't have enough space. Over time this will fill up your drive with files you stored there.
For information on how to use your drive:
How to use a WD external hard drive with your computer