I have to say, as a portly comic myself, I'd have to side with James as if i was in that situation i wouldnt just take crap from someone because they are more famous than me.
I'm far from a fan of Corden, but trying to get one over on a comic by making fat jokes , he really only left him one way out. James was aggressive rather than quick witted which let's him down but he had to react somehow.
All you guys saying he's crap etc and needs teaching a lesson, you have to remember comedy is subjective, not everyone likes the same stuff. Patrick Stewart came across as a bully IMO.