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.