Looking at the CSS variables page it seems that this is because they scale up the base font size at various breakpoints, starting at 106.25% at 576px, and going all the way up to 131.25% at 1536px. It definitely feels too heavy handed to me. The first time I viewed the website on mobile it seemed fine (because it's actually at or close to the browser's standard font size) so I wasn't sure what the problem was until I viewed it on my desktop. I can kind of see what they're going for as it keeps the layout fairly consistent, but I don't think it's the right approach.