Files sizes on external hard drive shrunk

I have a 4TB external hard drive that I keep a lot of pictures on in many folders and also a lot of pdfs and other book and document files in multiple folders. I was gust getting ready to back it up on another hardrive and I noticed the size of the files I had stored on it shrank. For example the last time I backed up the drive the book file was 487GB and now it size is 309GB. I added files to it since the last back so it should have more that 487GB of info on it not less like it is showing. The photo take up 1.16TB and there seem to be no changes to the folder size with the photos. My WD passport says used spade 1.71TB Free 1.91TB. What could be going on? At first I thought maybe the hard drive went bad and lost info so I took a different laptop and checked an earlier backup to see if it looked ok and the same thing it showed shrinkage, then a grabed a 3rd laptop and checked my third backup and it was a different brand and same thing. I do not know what is going on or how to fix the problem

 

Files are missing (check the number of files in the folder(s), not just the size used), the files have been compressed, or there is a HDD fault.

The HDD fault could show that files are missing, or the files are there, but they are likely no longer useable and likely reporting the wrong file size (thus the total is less).