Actually, I guess I was wrong about node_modules not being included in the slug -- it's just that the slug is compressed so node_modules don't take up anything close to their stat size. One common gotcha is that if you have large non-code files in your git repo (binaries/images/gzips/etc), those will end up in your slug and they don't compress well so they'll add a lot of bloat. If that's the case you can use a .slugignore file[0] to tell Heroku to exclude them. Good luck!

[0]: https://devcenter.heroku.com/articles/slug-compiler#ignoring...