At least in Firefox the preview of the edit font in the preferences (e.g. https://de.wikipedia.beta.wmflabs.org/wiki/Spezial:Einstellungen#mw-prefsection-editing) is smaller than usual for monospace, for me the preview has 10.5px, while on editing pages 12px is used (12.3px in NWE).
As mediawiki.editfont.css has just

.mw-editfont-monospace {
​  font-family: monospace;
​}

https://phabricator.wikimedia.org/source/mediawiki/browse/master/docs/uidesign/monospace.html explains why the font is too small in the preferences, though it remains a mystery for me why it has a good size on edit pages. Anyway, the size on edit pages should be kept, and the size in the preferences be increased to match it.