I can’t think of anything else it could be! I mean, when you compare season 1 Nathan to season 2 Nathan, he looks much more angular in season 2:

He looks a little less skinny in season 1 than he did in Cherrybomb (his legs were like sticks in that film omg), but then he got leaner again in season 2. Either that or it’s the body language thing confusing me - he always looks different in every role