A few weeks ago I took a business trip to San Diego. While there I was able to so some sparring with a friend of mine. He is a blue belt (iirc) in Brazilian Jiu Jitsu. We are both about halfway home to getting our black sashes/belts in our respective arts.
Instead of sparring mma, we sparred each other in our specialties. I was amazed at my opponent's grappling skills. Then again, I have no clue what to do on the ground. I did OK at first because I have good conditioning but eventually I would lose my balance (due to his maneuvers) and he would get the better of me. He was like an anaconda - slowly creeping his way into a submission hold of some sort or another.
But when we stood up, he was like a fish out of water and I was home again. My opponent had no clue what I was up to and fell into all of my setups just like I did on the ground. It didn't matter what range we were in, I dominated in the standup game.
We got a good sweat on and taught each other a lot of tricks.
Later, we had a discussion about standup vs. ground. He noted that in his gym the standup guys think they would beat up the ground guys since they wouldn't let the match go to the ground. The ground guys thought that they would beat up the standup guys since the first thing they would do is bring the match to the mat. Interesting discussion, but that is what mma is all about.
A better discussion, imho is how any of us would do in a street situation. And the answer to that is that we would all kick ass, as long as the opponent is not armed with a knife or a gun. Typically, anyone that regularly attends a gym is disciplined and in shape.
I would imagine that a large percentage of street fights are started by someone who is intoxicated in one form or another. These guys are usually not disciplined, nor in shape. Having a good base of conditioning is a huge advantage. I don't fear the biggest guys anymore - if I can get out of their initial flurries, usually my conditioning will win the day. Either I can fight until they run out of gas if I am in a corner, or I can simply run away and they will never catch me. I mean, I spar with guys who get paid to destroy other humans inside a ring or cage and who have very tight technique and conditioning - why would I fear some schmo on the street, even if he is taller and/or bigger than me?
In the end, outside of running away, my best defense is knowing my situations and environment.