Because size does not determine the outcome of the fight. It does not guarantee a victor.
An immature child against an adult the odd favor the adult.
However take a skill, trained martial artist against a much bigger person I'd say the odds favor the skilled individual/ The skill/technique will negate the the so-called size advantage
Superior technique has the advantage of size and strength.
There are weight classes in sports. Even in weight classes the superior skill wins. This is why Manny Pacquiao, Floyd Mayweather, Roy Jones, Roberto Duran, Sugar Ray Leonard, Thomas the Hitman Hearns, Michael Spinks, and others have been able to move into different weight classes and still win.