I had poor sleep for ages and was never overweight, which is true of most insomniacs I imagine. Sleep apnea may facilitate obesity but it doesn't cause it. It's calories in, calories out.

I think the strong correlation of obesity with sleep apnea however should lend credence to the idea that it at least exacerbates it. But more to the point, there are conditions that are very similar to sleep apnea but are misdiagnosed as such. And these may very well be more related to weight.