The website has basically no CSS, so it's up to your user agent to decide on its default presentation.
Just so happens that modern user agents don't really care about bare HTML, just CSS and JavaScript, so your default presentation is optimized to 1990s hardware at 800x600.
I guess "reader mode" is slang for "what the user-agent should be doing to make documents reasonable for modern displays", so press that button in your user-agent.
I'm usually just in reading mode, but occasionally I turn it off to see a/the blogs design because I feel it gives just a little bit more context to any post of text (for example, the author has a playful design and it gives me a small idea of their character). Unfortunate it's hardly readable, but it still says something about the author. HN is fun :)
Just so happens that modern user agents don't really care about bare HTML, just CSS and JavaScript, so your default presentation is optimized to 1990s hardware at 800x600.
I guess "reader mode" is slang for "what the user-agent should be doing to make documents reasonable for modern displays", so press that button in your user-agent.