Midface and the airway

The maxilla makes up more than 60% of the nasal cavity's structure. 

Good horizontal development (width) of the maxilla directly widens the nasal floor and increases the distance between the nasal walls and the septum. It keeps the palate broad and low, maximizing nasal volume and making the tongue-to-palate oral posture (and lip seal) and therefore nasal breathing effortlessly natural.  In contrast, in a narrowed maxilla, the high, narrow, vaulted palate protrudes into the nasal space, restricting airflow.

A wider nasal passage makes breathing through the nose easier and more efficient.

A well-developed, wide maxillary arch provides a spacious parking for the tongue to rest in its ideal position (in a negative pressure vacuum hold)

When the maxilla is wide, the tongue can rest against the roof of the mouth (correct oral posture). This upward pressure further stimulates healthy jaw growth and keeps the tongue from falling back into the throat.

If the maxilla is narrowed, the tongue is often forced down and back toward the oropharynx (the back of the throat). This "crowding" of the throat increases the risk of airway collapse during sleep, which is primary cause of obstructive sleep apnea (OSA).

By making nasal breathing effortless, good maxillary development prevents chronic mouth breathing, which is linked to various, cascading health issues.  

Chronic mouth breathing often leads to further narrowing of the face and jaw, creating a "negative feedback loop" that progressively worsens airway health

A retruded maxilla (one that is set back or underdeveloped) significantly compromises the pharyngeal airway by reducing the skeletal enclosure that holds the airway open. This structural deficiency creates a crowding effect in the throat, directly increasing the risk of obstructive sleep apnea (OSA). 

Because the maxilla forms the anterior (front) boundary of the upper airway, its backward (recessed, or retruded) position physically encroaches on, and restricts the passage

The soft palate is directly attached to the back of the maxilla. When the maxilla is recessed, the soft palate is also pushed further back into the airway, making it more prone to vibrating (snoring) or completely collapsing during sleep (velopharyngeal collapse). 

Since the maxilla dictates where the lower jaw fits, a retruded maxilla often forces the mandible to also sit in a recessed position. This further pulls the tongue and attached soft tissues toward the back of the throat (secondary mandibular recession)

The physical narrowing caused by a retruded maxilla changes how air moves through the throat: 

As per the Bernoulli Principle, air must flow faster through a narrowed airway to maintain the same volume. This rapid airflow creates higher negative pressure, which acts like a vacuum, sucking the walls of the pharynx inward and causing them to collapse, especially during sleep.

Finally, there are implications on posture. People with a retruded maxilla often reflexively tilt their heads forward and upward (forward head posture) to force open the restricted pharyngeal space