I don't want to seem rude, but to my ignorant eyes and those of its users, it's a forum software that requires a 10GB hard disk to install, and 20GB to update. I know that there will be a lot of technical details that will justify (or not) why, but, hey, that's a lot for a forum and there is no sane alternative way to install it.
I've found that I'm not alone: https://meta.discourse.org/search?q=disk%20space
There are entire pages of users complaining about the same or related problems.
At the end of the day, it's (almost) free... so what are we complaining for?