I have a client who has uploaded many giant images to the CMS (8mb+). I'd like to free up that space now that those image have been transformed to more web-friendly sizes. Is it safe for me to delete the original files stored on the server or does Craft require those to create new transforms?
Does deleting an original asset but keeping the transform folder asset make Craft replace the original file with the biggest transform?
No - Craft will always use the originally uploaded asset as the "source of truth" for generating new transforms.
You can, however, replace the originally uploaded 8mb+ file with a smaller one, and it'll be used as the source of truth for generating new transforms.