I was just looking at a picture of Vitali Klitschko knocking out Shannon Briggs (big muscles--his arms were twice the size of Vitali)
Vatali is 6.75 inches , weight 252 lbs with a record (Oct 2010) of W44 L2 (40KO's)
Shannon 6.4inches, weight 260 with a record then of W51 L6 (45 KO's)
Klitscho not only knocked him out, but Shannon was taken to the intensive care unit at the hospital in Hamburg Germany with a left orbital fracture, broken nose, torn left bicep, fight facial fracture and a burst ear drum.
If you had stood them side by side, you would have probably bet your money on Briggs based on his size and muscle mass alone. Even if Shannon had been shorter and Vatali taller, or the sizes reversed, the outcome would have been the same. It's not the size...it's the skill of the fighter that wins.