Even at my heaviest I was never fat in dreams. Even now there I am no thinner. As best I can tell it's an unusual flavor of body dysmorphic disorder - unusual in that I've never 'obsessed' one way or the other about my body size or shape, I simply don't have a good perception of it. My mental body concept does not accurately reflect my physical presence.
Parallel to this comes the memory of years of routine, of localized impressions from repeated acts of dressing, bathing, sitting, moving... These are what is contravened by the evidence of my senses now - bone where there was none before, a change in reach, slackness of skin.
I am still a stranger in my body, but the territory is easier to live in.