Ok so there had to be one.
I think several things are clear...
1)Dorn is pretty tall, but doesn't have an overly large frame.
2) He was padded over the years, this padding seemed to increase in later seasons.
3) Dorn filled out a bit, but not to excess on his own over the years.
What seems likely is that Dorn, who does look somewhat smaller these days, decided to trim down a bit. I don't see anything to suggest he's ill, but know one seems to know for sure.