Overview

The resource class of a job is not necessary to explicitly declare in your config.yml. If a resource_class is not explicitly declared for a job, CircleCI will use a default resource class size.

Please note that defaults are subject to change. It is best practice to specify a resource class, rather than relying on a default to ensure that your builds continue to complete if we downsized these defaults, and to avoid any unnecessary credit consumption if they were to be sized up.

Why does one project default to Large while another defaults to Medium?

The project's creation and set up date is the underlying factor for these defaults when they are not explicitly declared in the configuration file.

While projects created today will default to a Large resource class, older projects created before 6 December 2021 will continue to default to the Medium resource class unless explicitly defined to use another size. 

Additional Resources:

• Documentation: Resource Classes
• How to set the resource_class for docker and machine images