Change effective on 22 May 2017
The maximum slug size limit was increased from 300 MB to 500 MB. Once a slug size reaches 300 MB, Heroku will warn about larger slug sizes having the potential to cause longer boot times. The change was made to accommodate certain apps that may require more and/or larger dependencies.
As always, we recommend the best practice of keeping slug sizes as small as possible for the fastest developer experience in CI/CD, boot times, and other operations in the developer workflow.