A CMS, fundamentally, is a server application that pulls things out of the database, puts them into templates, and spits the resulting HTML out to the browser. There is no inherent reason why a page that comes out of a feature-rich CMS has to be bloated.
Under that definition of a CMS I agree, however I think what you are describing is an MVP of a CMS that would require additional features and (likely) bloat to truly be useful to most publications.
Examples:
Social Media integrations
Subscriptions integration
Advertisements + trackers (which will likely pull in additional garbage)
Analytics (GA, etc.)
Comment section + moderation tools
Media: images, videos
Font(s)
Responsiveness
It CAN all add up fast unless someone is keeping track and responsible for keeping these sites lean.
