With obstructive apnea, it's mostly hereditary. You inherit the shape of nose from your parents, why not the structures in your airway? Weight gain can make it worse as those tissues also can expand, and any weight on your breathing can help turn a partial blockage into a full one. That weight can include muscle weight, and body builders often suffer from apnea.

But the thing to keep in mind is that obstructive sleep apnea isn't so much a condition as a description of what's happening. Apnea means you stop breathing, obstructive means something is blocking it. Maybe it's because your fat belly is weighing you down, maybe it's your tiny nostrils and deviated septum, or maybe the back of your tongue is thick because your dad also had a thick tongue.

That's not even including self-imposed factors like central nervous system depressants (alcohol) and smoking, both of which can make apnea worse. Ever notice how drunk people snore?

Beyond physical obstruction, sleep apnea can also be neurological which is the more annoying one since only air pressure seems to fix it. You simply stop breathing.

Obstructive apnea doesn't need to be caused by fat either. I always thought sleep apnea was a fat person's problem until I realized at age 30 that when I slept on my back, the tissue mass at the base of my tongue was dropping into my airway. And I'm fit/slim and also a nasal breather.

(I used to think I always woke up gasping for breath in my 20s because I had frequent nightmares. Ugh, wish I could go back and fix it sooner.)

One theory is poor facial structure that develops due to faulty breathing technique.

Reference https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4686746/

The most common cause of sleep apnea is anything that obstructs your airways. Being overweight does that and is far and away the most common cause of sleep apnea.

This is such an often repeated trope, but it's not at all as clear cut as that. Sleep apnea *causes* weight gain, the arrow of causality can go both ways.

It's not exactly common, but you can have central sleep apnoea instead of obstructive sleep apnoea. Essentially, rather than your airway being blocked, you just stop attempting to breath for a period of time. I was diagnosed with this a few years ago, and IIRC there are multiple possible causes of it.

One theory blames "modern life" as a supporting factor, that since modern people eat more processed and softer food, that jaw muscles are out of proportion to the bone structure and this contributes to apnea, as well as tooth problems like impacted wisdom teeth.

https://news.stanford.edu/2020/07/21/toll-shrinking-jaws-hum...