Hmm – what I tried to do is look at the size of the main navigation, then reduce the sub-navigation text by 1px, which works nicely on smaller fonts.
When using larger fonts, that 1px hardly makes a difference, so I guess I need to re-think how I size it. Maybe a new option in Typography is needed.
For now, you can tweak the CSS.
.main-navigation .main-nav ul ul li a {
font-size: 17px;
}
You can tweak the pixels to your liking.
If you don’t know where to add the CSS, a plugin like this should help: https://wordpress.org/plugins/simple-css/