Best examples of overcoming a significant size and power disadvantage

Loughran vs Carnera and Baer.

 

That's his fight with Oakland Billy Smith, which was a great win. However, here is another one which really exemplifies the OP

"Charley Burley spotted Jay Turner of Dallas, 68 1/2 pounds of fighting weight, but pounded the Texan into a helpless hulk in six rounds in a scheduled 10 round bout last night. Burley, formerly of Pittsburgh and now of Minneapolis, entered the ring at 151 pounds, Turner weighing 219 1/2." -Associated Press

 

Here is another along the same lines:

"Tony Mundine added the Australian heavyweight boxing title to the middleweight title he already held when he stopped defending champion Foster Bibron here tonight. The referee stopped the bout in the 11th round when Mundine, who was conceding over 40 lbs., put Bibron down with a crushing punch. Despite being outweighed, Mundine was a clear pre-fight favorite and was well ahead on points when the bout was stopped. He now hopes to meet Bunny Sterling of Britain for the Commonwealth middleweight title." -Reuters

 

Billy Conn came within a whisker of an epic win over Louis.

Young beating Foreman also comes to mind.

 

Good man urself. I personally was convinced Canelo would win due to weight.