I've had the same sensation as well and tried to explain it to people, who thought I was insane for experiencing such a thing.
As said by several people in this thread, it starts when you're about to fall asleep, you feel like an enormous density that is being shrunk to a point in space and at the same time you feel like you are getting larger. It is like a disconnection of your body and brain.
It is not paralysis, since I am able to move during the experience, although that will disrupt the sensation (so I usually try to lay still to prolong the experience, it's so weird and beautiful at the same time).
I don't think it is a dream, since I'm conscious of where I am and that I can move. I can be in the sensation, feel completely detached. At some point the only thing that is left is the sensation of deep breathing, everything else will be gone (like the Never Ending story, where the Imagination world has disappeared into nothing and only the consciousness of the boy is left). When you reach that point it is somewhat unnerving, so to check that I am still there I move an arm and start becoming aware of my body after which the sensation slowly disappears.
I also don't think it has anything to do with migraines, although the brain areas that are affected may be the same.
I think it is an experience that you are are conscious of the first phase of sleep, when you'd normally be unconscious. It must have something to do with the deep relaxation of your body, the shutting down of parts of your brain in preparation for sleep.