Not more content, better content.
Content is words. Content is pictures. Content is video, captions, audio, animation, graphs, maps, tables...
Content is everything that you choose to put on a webpage. It should help your users do something.
Content design is about choosing and creating relevant content for your users. It's about giving them what they need in the most efficient way.
It's about letting your users do what they need to do, and then get on with their day.
Take a look at this page.
There's the heading, then a block quote from Sarah Richards, sub-headings and some text. There's an image.
Hopefully, the content helps you to understand the ideas of "content" and "content design".
The block quote stands out, and it's easy to read. Because we want you to understand that message clearly.
There's plenty of space between paragraphs to make things easier to read.
And that's all that content design is:
Finding the best way to present what your users need.