For McMahon to consistently be good at international level as a 6 or an 8 he will have to be more effective with his size than 95% of international players in the modern era have been. To expect that of him would be foolish. I'm not saying it's impossible. But I am saying it's not likely. Not because he's not good, but because it's not likely for a player to be better than most players before him.
I wonder if he's just had a good year. Much like Ed Quirk in 2013 but really lacked the physicality and at 108kg lacked a little of the bulk to perform as well in 2014 when the weight of expectation look far too heavy on him.
I in fact haven't advocated him adding weight. I've simply said his smaller frame (than his contemporaries - at 178cm it's certainly a bigger frame than mine) doesn't really lend itself to that of a typically successful 6 or 8, as he lacks the height to be a truly top line jumping option, lacks the bulk in collisions and to add to the scrum of many others, and that adding bulk would negatively impact his line out jumping.