Not sure rational logic can begin to explain this outcome
I think something must have been amiss with JoshuaThe guy has hard 33 wins and 1 loss, so he wasn't exactly a terrible boxer.
And it's about skill and power etc, not physique. So providing he has the required skill, power, and stamina there's no reason he couldn't beat Anthony Joshua
However, whether Joshua went into the fight expecting a walk over, didn't take his opponent seriously etc is something we might never know.