Yes, the cache ends up being bigger than the content database, but for text content that's typically not a problem. The human effort to type some text always hugely exceeds the cost of a few kilobytes of flash to store what they typed in a ready-to-serve form.

The generation process of taking the raw text and assembling the page around it is typically rather expensive for most CMS systems. Sure - it isn't theoretically expensive, but unless you want to engineer a CMS from scratch most people just pick one off the shelf and then end up having to pay the CPU time overhead of wordpress etc.