I have a Discourse directory installed on Digita Ocean, it has only 600 members and 160 topics and it already has 13 GB in Digital Ocean, would you like to know how I can help reduce consumption?
Assuming you mean, “how can I help reduce consumption?” (rather than this being the teaser for a future howto of your own), the answer is: figure out what’s taking up the space, and if it isn’t needed, delete it.
To answer the question in your title: Discourse takes up a lot of space because it’s significantly easier for everyone if we just ship a copy of everything above the kernel in one neat little package, rather than trying to shoehorn everything into whatever crazy setup people already have on their machine.
Be sure to read the topics here on how to clean up disk space. I would not recommend running an active community with less than 20gb and ideally 30gb disk space.