He never said that size equals fighting ability. He said that size is a massive advantage in a fight and he is right.
If size was irrelevant in a fight than why are there weight classes in all combat sports? If strength was irrelevant in a fight than why do all fighters attempt to increase strength as part of their training routine?
You are confused aren't you. although size can offer advantages some of the time there is a cut off point, being incredibly large is actually a disadvantage as the heart and lungs can't maintain the cardio needed for an intense fight situation. Within a very short period of time such a fighters muscles will be fatigued and burning with lactic acid and the fighter then becomes extremely vulnerable and will be unable to defend himself properly.
And fighters aren't just trying to increase strength for it's own sake, otherwise they would just all powerlift and become as big as they can, they are trying to increase their pound for pound strength and explosiveness, they are trying to become stronger for their size. A pro fighter would never compromise other essential elements of his fighting game just to become stronger.
Your statement that size is a massive advantage in a fight is naive and wrong, like I said, it is actually a huge disadvantage. But those with little to no fighting background continue to perpetuate this childish myth.