The vast majority of web designers are clueless or just don't care about font size recommendations. Most studies recommend at least 18 px for body text, but almost every website goes smaller. Medium is a notable exception, they have sane defaults.
The author might never have tested their website on a large display like ours and thus it'll look a bit odd and we have to zoom in.
Gets me from https://screenshots.firefox.com/2Gu0kninnhAnZKVw/www.smashco... to https://i.imgur.com/tld6Kl6.png
https://addons.mozilla.org/en-US/firefox/addon/automatic-rea...
https://addons.mozilla.org/en-US/firefox/addon/auto-reader-v...
It's an impossible problem to solve because sizes are given in pixel or percentage. Whatever you try to adjust, it will be utterly wrong on some combination of content and display.
Maybe fine when reading in portrait on a tablet or like, but not at all fine on a desktop.
This particular site starts with a relatively small font and not responsive which I'm sure makes it a pain to use on small displays.
Reader View