I run this user script in Firefox using Greasemonkey. It will also run using Tampermonkey in Chrom[ium] as well. It might work for you until the CSS is changed. It might not.

  // ==UserScript==
  // @name        hn_resize_subtitle
  // @namespace   com.kludgecode.hn.demo
  // @include     https://news.ycombinator.com*
  // @version     1
  // @grant       none
  // ==/UserScript==

  window.$hn_subtext_text_target =  document.getElementsByClassName("subtext");

  for (var i = 0; i < $hn_subtext_text_target.length; i++)
    {$hn_subtext_text_target[i].style.fontSize = 12;};

This, I believe, is a systemic [accessibility] problem with any browser, since day one.

They all should have additional settings:

1. minimum font size

2. maximum font size

to solve this kind of problems once and for all. I am actually very puzzled why nobody ever thought of that.