Regardless of the spin argument - "what if they were smaller" - it's obvious size is an advantage. So it's reasonable to question whether or not big men would have been as successful without their advantage.

There are currently 1085 pro heavyweights. Weight limit - 200-300lbs.

There are currently 4138 pro fighters between 147-160 - 13lbs difference.

The competition is clearly much more fierce the closer you get to an average sized man.