A step that is expected to not use much memory uses more.
The step is not the only one using memory, parallel steps and/or services will be using memory at the same time.
Confirm that all parallel steps or services are relevant to this step and disable the ones that aren't. Consider increasing the size of your instance, if all steps are required.
The warning message does not lead to an issue with the build, and instead serves as a warning. If it does run out of memory, there should be a clear error displayed.