Tommy Burns at 5'7 and 180lbs. was the shortest Man ever to hold the Title. Known as a good boxer with a good punch,Tommy took on bigger Men{not hard at 5'7} and defeated them,until he ran into Jack Johnson.Leon Spinks, at 6'1 and 200lbs. of course pulled the upset of the Century by beating Ali,was an aggresive,balls to the Wall fighter,with limited defensive skills and not to much punch. I believe Tommy could control the action in this one,by boxing, counter punching, controling the clinches, and a steady body attack working Leon down to a 15rd. U.D.
Leon Spinx talent is a bit underated. he had skills, movement, etc. He was just not their mentally and had no whiskers
Still even IF Spinks was underated, I still cant see him beating Tommy Burns who was a great fighter in his own right(In a pound for pound sense.)