On the contrary, I've always found HN's font size to be one of the worst parts about the UI. I actually have a userstyle set to override it

  body {
      font-size: larger;
  }
  
  .comment, .default, .comhead {
      font-size: 14px;
  }
  
  .subtext, .pagetop {
      font-size: 13px;
  }
  
  .title {
      font-size: 16px;
  }

It's not the prettiest CSS or the most well done, but it works and it's better than the defaults imo.

I do use HackerNews at 150% because otherwise it's unreadable