Works fine for me, as you can see:
I opened File Explorer and typed size:gigantic into the Search this PC box, then pressed the Enter key. What exactly happens when you do that? (BTW: It takes a minute or two for the search to complete.)
Most or possibly all of your 'gigantic' files are normally hidden from view. You can make them visible by going to the View ribbon in File Explorer and putting a check mark in the Hidden files box, like so:
There are additional system files that aren't displayed even with a check mark in Hidden files. I can show you how to find them if you like.
You can help File Explorer find files by indexing your hard drive - like the index in a book. Go to Control Panel > Indexing Options
Is there a particular 'gigantic' file that you think you should have but aren't finding?