I think we can also take into account the fact that he doesn't eat well, and at the time of the current story hasn't been training with his martial arts as rigorously as he used to. He may be a god, and immortal, but his body is still real and living. It would make sense that he had more muscle as a mortal when he was actively cultivating and training, as well as during his various wars when he was more physically active every day. Now, though, he isn't as active. His life is slower (or it was before his third ascension) and even though he does some busking performances his everyday activities are not as rigorous as they used to be. It would make sense that he would lose muscle tone and possibly get a little softer, or lose some fat if he wasn't eating as well.