How to use .slugignore to reduce compiled slug size on Heroku - cant see files

I am trying to deploy a machine learning app in Python on Heroku.

So I got this ‘Compiled slug size: 662.5M is too large (max is 500M)’ error that seems to be very common. I have read this help article, https://help.heroku.com/KUFMEES1/my-slug-size-is-too-large-how-can-i-make-it-smaller

but I can't see the list of files by size?

I run the following in Git bash on windows to see the files:

heroku run bash -a my-app-name

~ $ du -ha --max-depth 1 /app | sort -hr
4.0K    /app

any idea what can I do to fix?