Size has very little to do with winning a Fight when talking about going against trained fighters...the more skilled Fighter with proper technique has by far the upper hand....and Overeem is a very skilled fighter.
and in many cases the Bigger you are hinders your cardio...your works alot hardr to supply "Big Muscles" and can gass you out fast...especially heavy weights..
that's the reason most Elite Heavyweight MMA champions have been towards the smaller end (5'10 - 6'1 210 - 230lbs) instead of the Hulking Overreem size.
all of these Fighters are about the size of Jameer Nelson....fighting men 6'3 - 6'5" 260+lbs