Oh, the rabbit-holes I find myself in when browsing SE (and in general)...
Not sure if it's a Safari or Stack Exchange bug.. Chrome has the opposite problem.
When following a link on SE and then navigating backwards in Safari split-view (and maybe in normal, manually resized windows), the previous SE page is zoomed-out.
When navigating to a user-profile page on SE, Chrome split-view does not zoom out.
(sorry Dmitry Ulupov)