A lot of things contribute to sleep apnea, and it is more prevelant than most people realize. In most cases, it is not dangerous, but it certainly can be. It also causes loss of REM sleep, thus the sleepiness during the day with some who have this problem. In my sleep study, I was having about 20-30 "episodes" per hour, though it was only for a second or two...some people stop breathing for several seconds or longer, which can be life threatening in some situations.
Being overweight is one factor that contributes...the excess fat around the neck and throat when you are in a prone position and relaxed (sleeping) causes pressure on the airway. That's why it generally worse when sleeping on your back. Getting older (loss of muscle tension in the face/neck, and "extra" skin in those areas) is a contributing factor as well.
I still snore, but not nearly as bad as I used to. It is USUALLY worse in larger people, but not always...and a lot more women have it than will admit to it.
It is not something to take lightly, as it really screws up your normal sleep patterns, and the brain does not get the "down time" which occurs during REM sleep, which can lead to uncontrollable/unexpected falling asleep while driving, etc. It can also cause depression in some individuals.