If size in a fight matters so much regard that people always say a bigger person will always win no matter what then how do you beat them?

Why I ask is because I train in mma and I doubt myself and skills all the time and I'm afraid that mma wont work for self defense because everybody on the internet keep saying that sparring in real fighting are two different things and that sparring can not compensate for real fighting and thus it won't... show more Why I ask is because I train in mma and I doubt myself and skills all the time and I'm afraid that mma wont work for self defense because everybody on the internet keep saying that sparring in real fighting are two different things and that sparring can not compensate for real fighting and thus it won't work in a real life situation against a bigger opponent. All these people on the internet with their article keeps saying that no matter what martial arts,speed,skill,smarts can never overcome strength,size,power and brute force so regardless the odds will always be in a bigger opponents favor and you have no chance to beat them. So tell me some how do you beat such a big monstrous opponent since no matter what you cant beat them because due to their size making them invincible. I'm scared in scared that when the time come I will fail if it comes down to it. How do I know my boxing,kickboxing,must thai,karate,etc will really come in handy if everyone on the net keeps saying so?
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