Mobile browsers are assuming you're looking at a legacy page optimized for desktops (widescreen) and have a relatively large virtual screen size by default. They expect you to manually zoom in as necessary. Adding this helps:

  <meta name="viewport" content="width=640, initial-scale=1">

This matches the max-width specified by the CSS. However, a smaller viewport width might be appropriate to increase the text size on mobile.

Safari reader mode on mobile works great. But then again, this is a site where you should not need it.