System Info says my Documents folder is 170GB when Finder says it's only 8GB!
Documents in the System Information pane doesn't mean files in the Documents directory.

It means files that are not other, already defined media types - ie. not photos, books, movies, etc. This would normally encompass the sort of files you'd put in your Documents folder, but they don't have to be kept in there to be part of that total.

If you actually click into Documents on that System Info screen, and then hit the File Browser button up top you'll be able to see what directories are using up the space.

Personally, I do see it's finding a large (40Gb) amount of space used in Library, which is often where things like Google cache data. Apple also caches a lot in Library.