Right now the tab font size is applied to the individual li elements in the vector menu list in Legacy Vector and not at all in normal Vector.
Issues
In Legacy Vector, this causes the tab font size to be applied to submenu items multiple times, making the text smaller for submenus.
In normal Vector, the tab font size is not applied to any menu items, causing menu item text to appear larger than the tab text.
Cause
Both of these issues were 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 anywhere. T287052 tried to fix this for Legacy Vector only, but it caused the above issue with submenus.

