Purging the cache fixed it.
Install the Heroku Repo plugin:
https://github.com/heroku/heroku-repo
Then run:
heroku repo:gc --app your-app-name
heroku repo:purge_cache --app your-app-name
These commands will execute git gc --agressive (git-clean) and delete the contents of the Heroku build cache stored in your application’s Git repository.
Doing this on a recent app reduced my slug size by 100M.
Src: https://robots.thoughtbot.com/how-to-reduce-a-large-heroku-compiled-slug-size
👍 thanks