I would argue that muscle can still be needless weight...
Yes but it's his physique and power that make him such a difficult player to play against, combined with his skill on the ball of course.
There's not many players in the top leagues who beat a man so easily and so often as what Barkley does. Any pros that I've seen asked about him (who have played against him) have commented how difficult he is to shirk off the ball.
He's started every game this season, so his stamina is obviously not an issue. His job isn't currently to track back. He's there to be our main source of attacks...