With good breathing, the air traveling through the trachea reaches a wider area in the lungs (top and bottom) where it is distributed to the alveolis. Shallow breathing, by contrast, draws minimal breath to the chest area, which means that the air will mainly reach the upper part (apex) of both lungs.
When we take shallow breaths, we have shorter inhalations and exhalations, albeit at an equal cadence (shortness of breath by contrast indicates shorter inhalations than exhalations), and as a result the air is concentrated at the top of the lungs.
Ventilation (gas exchange: O2 in, Co2 out) is greater at the base of the lungs than at the apex for two main reasons. First, alveolis are less expanded (smaller volume) which makes them more distensible and therefore capable of more oxygen exchange. Second, gravity facilitates increased blood flow at the base because it is pulling blood down towards the base.
So, for efficient ventilation the air should end up in the lower parts of the lungs. If you take shallow breaths, you are forced to compensate by taking more breaths per minute, because you are using the wrong muscles. Chest muscles were not designed for breathing, your diaphragm is the only breathing muscle and the most effective one at that.
If the diaphragm is under-used, it will become tense and weakened, the effects of which will be obvious during exercise as you get frequent side stitches or get tired quickly.
Sources:
Powers KA, Dhamoon AS. Physiology, Pulmonary Ventilation and Perfusion. [Updated 2021 Aug 20]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539907/
Singh UP (2018) Chronic Hyperventilation: Hypocapnia and Psychophysiological Dysregulation . Psych Clin Ther J 1: 104
https://www.lung.org/lung-health-diseases/wellness/breathing-exercises
https://www.consciousbreathing.com
https://blog.powerlung.com/better-breathing/the-dangers-of-shallow-breathing
BONUS!
EXERCISE YOUR DIAPHRAGM
The goal is to extend the exhale and squeeze the air out.
1-Sit comfortably with relaxed neck and shoulder muscles
2- Breathe in slowly through your nose
3- Purse your lips and exhale slowly through your mouth
OR
Press your lips together making an “sss” sound
4- Repeat as often as possible and practice as frequently as possible until you notice a change in your breathing pattern