My dog is 50-50. Sometimes on her side, all stretched out and sometimes curled up in a ball. Depends on what type of surface she is sleeping on. The couch & crate, she is always in a ball. The floor, always on her side. My bed & her beds are 50/50.
I once read that when a dog sleeps on their side or back, it means they feel comfortable because they're exposing their internal organs, making them vulnerable. A dog curled up in a ball is protecting their internal organs from predators. That's not to say that every time your dog sleeps in a ball, they feel threatened. I'm sure a domestic dog sleeps for comfort, but you probably wont ever come across a wild dog or wolf sleeping all stretched out.
