I got diagnosed with sleep apnea and asked about this, since I'm not overweight. The doctor said it's more about the structure of the mouth and throat than obesity. There is an area at the back of the tongue that gathers more fat and can block the throat, but palette structure is a big factor. Along with jaw size and throat size.

I remember reading not too long ago on HN an article about our dental health in modern times: https://www.scientificamerican.com/article/why-we-have-so-ma...

It could be our diet contributing to smaller jaws/palettes that increase the prevalence sleep apnea?