Jared Spool, writing for User Interface Engineering:
It’s when type size is combined with other attributes that suddenly the type might seem too small or too big in the design. And we can make the type size attribute work again by adjusting the other attributes and leaving type size alone. Sometimes, when we’re focusing on a specific attribute, like type size, the best way to make it feel right is to tweak other attributes.
This effect is what Gestalt theorists call field theory. The field is the holistic view of the world, or in this case, the full design. In Gestalt theory, you get to the best outcomes by adjusting all the attributes in the field and not worrying about a single one.
This is why experienced designers are always talking about how the type feels on the page. It’s not that they want to seem mysterious. It’s that they are talking about the entire field — the holistic view of the design.