Perhaps he had great hidden stats, perhaps you "rolled" a 1 and he "rolled" a 6, perhaps you used up all your luck beating on 278lbs fighters.
The simple fact is that when there's a 40lbs weight difference it's not "bullshit" when your fighter loses. It could be unlucky, but it's not bullshit.
Yes, it is bullshit if they're using a style they have weak stats for. It would be bullshit if we were the same weight and using the same style. It's ridiculous.