Theatre example from Police Cops In Space. After the two Police Cops blast their way through the robot army to face their leader, he sneers: "You may have taken out a thousand robots, but now let's see how you cope with...one robot." And in the ensuing fight he kills one of them, and would have won were it not for time-travel shenanigans.
Miyamoto Musashi said in the Book of Five Rings that "one man can beat ten men. Just as one man can beat ten, so a hundred men can beat a thousand, and a thousand men can beat ten thousand. In my strategy, one man is the same as ten thousand".
One of the few straight examples in Real Life is aerial combat. The larger the fighter formation, the more unwieldy it is, as problems with coordination and friendly fire mount. Smaller fighter formations, on the other hand, coordinate better, and against a large swarm of fighters can very successfully default to "Shoot everything that's not my buddies". And then you throw in things like superior equipment, superior skill, and AWACS support... For examples, see the failure of Big Wing tactics during WWII and the Israelis curbstomping the Syrians over Bekaa Valley.