Update: For me at least this turns out to be a user (me) error. The Stylus plugin is enforcing the older SE layout which used a 1.3 line-height instead of 1.5. In fact I railed against a 1.5 line height at the time.
I tracked down the rogue source of the
line-height: 1.3 !important; to an inline style applied to the page after it was fetched, and found the plug-in modifying the page.
There does in fact appear to be something going on with the
line-height attribute. Other text sizing attributes appear to be unchanged, but I'm seeing a line-height of 19.5px for SF&F, but 22.5px for meta.scifi and other SE sites.
22.5px appears to be what would be expected based on the CSS:
--s-prose-spacing-condensed: calc(var(--s-prose-spacing) / 2);
<div class="s-prose js-post-body" itemprop="text">
<p>Has a change been made to the default text size on the Sci-fi main site? I can't see any setting that I've changed, but everything is at a smaller size there than any other part of the stack exchange network, including here on meta.</p>
I'm not sure where the 19.5 is coming from.