Lets, take a look at our case of nose breathers versus mouth breathers.

For what reason do they look so different, when the hereditary makeup is hypothetically the same?
The answer to this question lies within the tongue. An amazingly incredible muscle, the tongue can be our closest companion or our biggest foe.

Why, you ask?

The body is made for breathing generally through the nose. Air can be warmed, dampened, and sifted as it goes through the nose.
With nose breathing, the tongue rests on the roof of the mouth. The tongue puts pressure on the palate. As we grow and develop, this acts as a natural expander to widen the upper jaw and counteract the pressure of the cheek muscles pushing inward.

With mouth breathers, the mouth is open and the tongue is constrained down and forward to permit air to go through the mouth and into the throat.
The tissue of the mouth, tonsils, and throat dries out and gets aggravated. The tonsils frequently augment, aggravating the issue.

This means the tongue is not putting pressure on the teeth and roof of the mouth. When the cheeks continue to push inward, it creates a narrow upper jaw.

This frequently brings about a long and narrow face, a retreated jawline, gummy smile, crowded teeth, and a larger than normal nose.