It's great that you've learned some street fighting and probably some preemptive strikes, but I don't think you've really been taken down and on your back with a big guy pounding you or you'd know just how difficult that situation is. A big guy (I don't mean heavy, fat & weak. I mean big, conditioned & muscled) is very much a danger. Muscle mass does matter. Strength and weight does matter in a 'ground & pound' situation.