Oh boy, another fighting thread. Size and strength alone do not a winner make. Curtis Lefler (RIP) used to get handled by a lot of guys at Relson's original academy. And he was stronger, and bigger than just about everyone here. Size does help if the person has balance, agility, and has some knowledge of fighting.
Not gonna argue styles, anyone who has any clue can see what is effective and what is not. Not gonna argue over mystic dim mak secrets to deadly for the ring/cage.....
All things being equal, bigger is best. But most of the times, all things aren't equal. I was getting killed by brown and black belts last nigth that I outweigh by 30 lbs and whom I have a decidedly LARGE strength advantage over. Their game is different because of their size, lack of strength. But if a guy plays more guard based (whether it be half, x, spider, butterfly) and he can sweep and get top, he neutralised the bigger fighters strength. Likewise with a good standup fighter with good footwork who can evade a bigger fighters "haymakers" and end the fight with a good three to five strike combo, all that size and strength is neutralised.
BJ Penn would KO pretty much ANYONE, anysize, and he walks at 175.