By themselves, individual people may be quite intelligent and capable of making rational, informed decisions. A group of people can be surprisingly easy to fool or manipulate by comparison. And when we scale up to "the public" ...This may be one of the most extreme cases of Conservation of Ninjutsu.
This trope may be averted in Real Life conflicts by the Lanchester's laws. with the important proviso that when all other things are equal, the numerically inferior side will suffer both numerically and proportionally greater losses than the numerically superior side... but things seldom are equal in Real Life - that is why quality can and often does prevail over quantity.
Aerial warfare. Only a little difference in quality - most importantly pilot training or tactics - can tip the scales on the side of quality against the quantity, resulting in Conservation of Ninjutsu. The most extreme example of this must be that over Karelian Isthmus 28. June 1944. Ace Pilot Hans Wind (75 victories) and Wingman Nils Katajainen (36 victories) attacking twosome a formation of 100 enemy planes and scattering it. (Both pilots survived the encounter).
This trope is sometimes combined with Plot Tailored to the Party. Multiple characters who attack a specific enemy may get their asses handed to them not necessarily because they're weak but because they're not the suited people to fight that specific foe. A token enemy can easily overwhelm a group of heroes, but a one-on-one will be much harder fight.
When 3 expert fencers take on 50 novice fencers, this trope seems to be in effect in that the expert fencers had great success even though outnumbered, until the the number of novice fencers dwindled and they suddenly became much more capable. Eventually, it is the novices who win.
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"