I don't know. "Muscular" isn't exactly the first thing that comes to mind when I see Mewtwo. He can appear pretty average-strength at first glance. If nothing else, he certainly doesn't look like he weighs 269 pounds. Nothing about him suggests a weight that high to me.
I also disagree on the second point. It may just be because the only time we've seen Mewtwo's weight have an effect on gameplay is in Smash, but it's hard for me to imagine him weighing 269 pounds. In my opinion, there's not really anything that feels wrong about Mewtwo being lightweight in Smash. It just sort of feels...natural, I guess. I'm sure that's because Mewtwo has always been a lightweight fighter in Smash, though, and I've just grown used to it. My next point may also help explain that feeling.
So, while Mewtwo being lightweight isn't necessarily canon, the idea of Mewtwo having extreme power and low defense is actually a grossly exaggerated version of his mainstream self. Mewtwo's strong suit in the main Pokémon games has always been his Special Attack and, to an extent, his Attack stats, while his Defense and Special Defense stats have always been his low point. Of course, neither of his Defense stats are truly low, but, relative to his other stats, they do fall a bit short. Smash's Mewtwo seems to be hyperbolizing that stat difference, which would explain his low weight, or "defense." It does make sense when you think about it, and, as I said before, I'm totally cool with it. I think Mewtwo being a glass cannon makes him more fun to play as. I do, however, wish he balanced a little better.