This thread contains several posts that have very long, unbroken, lines. If those lines had started with four (two?) spaces they've have got different CSS - the pre-formatting would have put the very long unbroken line inside a different box with scrollbars.
Edit: oddly, going directly to that comment looks better than going to the main thread. It's still smaller than normal on mobile, but nowhere near as bad as the main thread.
Here's what things look like for me in Safari on iOS:
Here's one example post: https://news.ycombinator.com/item?id=11610111