The pigs are pretty social critters. Even though they've got acres to roam, when you see them they're almost always hanging out with other pigs. This happens when they sleep, too.
Here four 400lb sows are sleeping with a smaller pig (look in the crack to the right of the photo) and two goats. I've been surprised at how close a relationship that the goats have with the sheep. they sleep with them, and spend most of the day out with the pig herd grazing with them. The boar is even a little protective about the goats. He clearly considers the goats to be part of his herd.
The smaller pigs fit themselves into the valleys between the sows. I've been really surprised that they don't get crushed. The two sows in this picture outweigh this little pig by at least 3-1. But I'm sure that it makes for warm, comfortable sleeping.