The posts are text only so the memory usage doesn't grow very fast. You can post images using Markdown but the images themselves have to be hosted elsewhere. I've created a few million posts on a development laptop and the memory usage was not bad (can't remember the exact number).
Right now there is no memory limit or handling for when memory reaches a certain point. Pruning based on FIFO or fewest replies could work, or OS level techniques to handle memory limitations like swap.
I will add some screenshots to the repo. Thanks for the suggestion.
The bigger problem is that the amount of information you can consume using RSS feeds is declining. Most sites don't publish RSS feeds of their content any more.
Sadly RSS is left over from a time when things were more open. Now everything is an app and everyone wants you to stay in their walled garden.