Every fight that I have been in my life except for one and the vast majority of fights that I have seen in my life, there was a big guy that thought he could fight simply because he was big but he lost to a smaller trained fighter. I see it all the time in training, too. Helio Gracie could tap all of the big cans. However, the truth kind of lies in the in-betweens; there is a saying that size stops mattering that much after heavyweight. You need some size. I know, if someone has an extreme size advantage on me, it really limits what I can do. If you had a guy that was 6ft 200lbs >15% BF above average athleticism who trained MMA for a year, he would probably win in a MMA fight against MM. Size can be less important in the street also.