Caused by: T287052
Right now the tab font size is not applied to menu items in the new Vector skin. I've split this from T288336 since that task was closed because vector is not responsible for supporting submenus added by gadgets and use scripts.
The tab font size is not applied to any menu items, causing menu item text to appear larger than the tab text.
The issue is caused by T191021's commit, https://gerrit.wikimedia.org/r/c/mediawiki/skins/Vector/+/702456/, which for some reason only applies the correct font size to span elements inside the links inside the menu items even though those span elements are never added by mw.util.addPortletLink.
We fixed this for legacy but not for modern. It may make sense to isolate some styles to the more menu itself in CSS.