For this simple wikitable, monobook has a broken rendering:

{|class="wikitable"
|-
|<span style="font-size: 600%;">ABCDEF</span>
|}

Please take a look at these two renderings:

https://en.wikipedia.org/wiki/User:Niboshi/MonoBookSkinTableLineHeight?useskin=monobook
https://en.wikipedia.org/wiki/User:Niboshi/MonoBookSkinTableLineHeight?useskin=vector

It seems the style .mw-body { line-height: 1.5em; } in resources/screen-common.less is the cause.

Tweaking the style to 1.5 (removing em) fixes the problem.
Perhaps this article is relevant: https://allthingssmitty.com/2017/01/30/nope-nope-nope-line-height-is-unitless/