Who do you think were some amazing undersized athletes in sports history

Brad Marchand comes to mind. At 5 ft 9 & 180 lb, he's been an impact player (who can be counted on for ~25+ goals / year) for the Bruins since 2010 or so, and is known for his agitating style of play.

https://www.youtube.com/watch?v=nocotQPTTsg