Could 5'3", 125 pound Demetrious Johnson beat up a massive NFL linebacker in a real-life bar fight??? 

That's what we asked the UFC flyweight champ ... and he had a pretty good idea how things would go down. 

Here's the deal ... DJ is considered one of the greatest UFC fighters of all time -- but we really wanted to know if things got violent in the real world, does skill trump size and brute strength?? 

Mighty Mouse breaks it down ... and warning: don't screw with this guy.