The Banner Saga has this as a core gameplay mechanic. The combat is turn based, but turns are alternating between individual characters of the two opposing sides one after another. This means if you have twice as many fighters, they have individually half as many turns.
A Magic A Is Magic A example. In the face of a horde of aliens that are both physically and technologically superior to anything Humanity has to offer, the Master Chief and the other Spartans are the only humans that are physically strong enough, technologically augmented enough, and technically skilled enough to take down thousands of alien warriors (many of whom are physically comparable to the Spartans). The legendary status of the Spartans is such that the military refuses to officially acknowledge their deaths, instead listing them as Missing In Action.
A dramatic moment at the end of the campaign of Halo: Reach sees Noble Six's final moments from the perspective of his suit's helmet camera, which he had just dropped on the ground; as a company of Elites closes in, he takes on seven of their best in an all-out brawl, killing over half before he bites the dust.
In the third installment in the Hearts of Iron series, naval combat is based around "effectiveness" percentages. These are affected by unit experience, weather conditions, supply shortages, commander skill, etc. For each naval unit that is added to a fleet, the "Disorganization Penalty" stacks higher, bringing down the effectiveness (and by extent ability in combat) of all the ships in the fleet. While the effect is negligible for small and only a minor problem for medium sized fleets, massive armadas (50+ units) may find themselves being absolutely massacred when up against only two Captial Ships and their escorts, as the Disorganization Penalty has been stacked so high that even the armada's battleships are seemingly worthless.
This can happen in online multiplayer FPS games such as Call of Duty and Battlefield when small teams are outnumbered by large contingents of players from the other team. While objective based game modes can be easily thwarted by numerical superiority, Team Death Matches tend to play out the opposite way. In, for example, a 1v12 Battlefield 3 TDM, the severely outnumbered player may well take down two or more players before he/she is spotted by the other team and eliminated - at which point he/she spawns randomly somewhere else on the map. This results in the lone ninja gaining substantially more kills in the target rich environment than the team of regular ninjas can simply fighting one player.
Played with in the Double Dragon series, most notably in the NES Double Dragon 3 and the SNES Super Double Dragon. Noteworthy in that one normalMook can fuck you up real bad if you're not careful. A bunch of mooks, on the other hand... well, they can still fuck you up real bad, but the gameplay provides several ways in which you can use their numbers against them, especially in Super Double Dragon - such as grabbing one mook and throwing him at another, tricking them into throwing their weapons at each other, crowding two or more mooks into a corner, thus limiting their attacks and enabling you to beat 'em all up at ONCE (a trick that works really nice when one of the baddies is a Level Boss), etc.
Bad Dudes Versus Dragon Ninja. Armies of brightly dressed Highly Visible Ninja rush at the one (or two) good guys in broad daylight then each one falls down (and vanishes) after being struck a single blow (in fact in the case of the chi punch several ninja can be killed by the same blow).
Dragonest has the Priest class which is exceptional when dealing with multiple enemies due to the large area of his spells (some actually hit more times if there are more enemies, especially Inquisitors) but utterly horrible when faced in single combat.
A noted problem with Metal Gear Solid 2: fighting thirty Metal Gears is significantly less dramatic than the usual finale of fighting one, because of the greatly reduced significance of each foe; in fact, Metal Gear RAY's require only a handful of missiles to destroy, while their REX predecessor (Which they were designed specifically to be able to defeat) required some 20-30 of those missiles and a lone ninja (himself taking full advantage of this very trope) to perform a Heroic Sacrifice.
No ninjas or robots either and applying to main characters, but still: Dante or Vergil alone can use their full powers in the first phase of the fight against Arkham. When the second phase rolls in, bringing Vergil or Dante (respectively according to character used) with it, the player loses his Style-based moves and Devil Trigger transformation, while the interloper also cannot fight at full power.
The Agni and Rudra boss fight from the same game pits Dante/Vergil against a pair of demons armed with enchanted swords. If you defeat one of them and don't finish the other off quickly enough, the survivor grabs the other demon's sword and starts Dual Wielding, unlocking some nasty combo attacks and becoming a much bigger pain to defeat.
In Spider-Man: The Game of The Movie, one level relies heavily on stealth, and if you are spotted or trip an alarm it brings out a couple Super Soldiers, giant robots that are extremely formidable opponents. Even one is a handful, and if you run into more than one, your only hope is to run and hide. A couple levels later you have to fight your way through dozens of Super Soldiers, which are notably easier to get past.
In Mega Man, the mass-produced Joes are basically Arm Cannon fodder. Only the unique Robot Masters are a challenge. Gemini Man from 3 himself follows this trope. He starts the battle by doubling himself, and only attacks with a weak blaster (in response to your fire) and by Collision Damage. Only when you destroy the clone does he break out the Gemini Laser.
In the Super Smash Bros. series, certain stages in Classic Mode pit you against a "Team" of a large amount of characters. The members can be flung off the screen with one solid hit - even heavy characters like Bowser blast off when part of a team. Meanwhile, some stages can give you hell with just 1-3 opponents and even with the very occasional ally. Entire game modes, such as the Trope Namer for Multi-Mook Melee, are based around this principle, giving you challenges such as defeating 100 opponents, or holding out for as long as 15 minutes against a never-ending tide of mooks. However, a devastating subversion is the aptly-named "Cruel Melee/Brawl", where every single mook is far more powerful than you, and they still spawn in three at a time.
Another Star Wars example, in Republic Commando, when Delta Squad (essentially the ninjas of the Clone Wars) splits up to take down the Core Ship on Geonosis, Delta-38 (the player's character) lampshades this trope, almost making it into a Crowning Moment of Awesome:
Delta 38: Alone against all these droids? Heh, they don't stand a chance.
Slightly older Star Wars example is Jedi Knight: Jedi Outcast, in which the player's initial skirmishes with the Reborn darksiders are virtually mini-boss battles, but as the game progresses and Kyle Katarn is pit against 3 or 4 at a time, the fights become easier. Not because the Reborns are weaker, but because both Kyle and the player are so much stronger.
Lampshaded with dark hilarity at the end of Max Payne. As he continues to gun down the Big Bad's Killer Suits in her penthouse suite, the PA system crackles to life:
"What do you mean 'he's unstoppable'? You are superior to him in every way that counts. You are better trained, better equipped, and you outnumber him at least twenty-to-one. Do. Your. Job."
The Spy, one of the nine character classes, has the potential to singlehandedly wreak havoc across the entire enemy team. A lone Spy can cripple defenses, take out high priority targets and even ninja-capture objectives when no-one's looking, all while causing paranoia among enemy teammates. Multiple Spies, on the other hand, don't work so well: if one Spy messes up, the enemy team will be alerted and will plan accordingly, making life much more difficult for the other Spies. They also tend to interfere with each other's plans, particularly when trying to take down the same target or Sentry Gun nest. When a team has three Spies or more, chances are high none of them is doing well at all, which in turn hampers the others' effectiveness, especially as Spies aren't cut out for head-on combat.
The Mann vs. Machine Mode: Six mercenaries, hundreds and hundreds of giant robots and tanks. Guess who wins? (Well, hopefully you.)
In Batman: Arkham Asylum and Batman: Arkham City, large armies of mooks present little more challenge than a single mook, except that they take more hits. But dodging/countering works the same. Just beware guys with knives and guns, while pummeling one to twenty men with baseball bats or their own fists for protection. On the other hand, when only one guy shows up in a room, you can bet it's either a Boss Battle or a Boss in Mook Clothing. More enemies can even make a fight easier because more enemies means more options. Don't wanna deal with the toughest enemy in the room? Just punch 5 easy guys once to build up your combo so you can use an instant takedown on the scary guy.
Ninja Gaiden: Ryu, a lone ninja, can take on a seemingly endless horde of ninjas, demons, and fiends of all sizes and colors - and the endless hordes of ninja that come after him can barely touch him. Granted, higher difficulties on the Xbox game require that the player EARN every iota of their ninjutsu.
This game actually has this as a player's power. The more enemies that are nearby (capped at 10 to balance things a little), the stronger a character possessing such a power will be in battle against all of the enemies. Also carries over to a few of the optional powers accessible to anyone, which can improve offense, defense or other stats across a whole team.
One such power, Rise to the Challenge in the Willpower set, not only boosts your health regeneration rate higher as more foes surround you, but it also gives those foes a medium to-hit debuff.
The second PSP installment of the Ratchet & Clank series, Secret Agent Clank, has a skill point challenge that references this trope. Titled "Inverse Ninja Law", it requires you to defeat 99 ninja mooks during a boss fight where they spawn endlessly, far, far more than you need to defeat to beat the level.
An early mission in boiled down to "Storm the enemy base alone. Have fun." Of course, it should be noted that your character is explicitly a Super Soldier.
And then of course you have Zack's final stand against practically the entire Shinra army. Despite eventually dying in the end, Zack canonically takes out all but three or four of them before finally reaching his limit. Based on what you see in the scene just prior, we're talking about single-handedly destroying a good 100-200 rifleman, backed up with missile-shooting battle copters, all using a single sword.
One of the stages in Disgaea: Hour of Darkness has you fighting a giant enemy (who is so big all you see of him is his foot) who divides himself into ten separate enemies. Love Freak Flonne lampshades this by saying its love is divided by ten. However, as noted by the Prinny commentary on the DS version after you lose, love is not a battle stat. Even though all of these divided enemies are a presumed to be a a tenth the strength of the original, they are still the highest level enemy in the story line next to the Final Boss. In other words, you're screwed.
Two examples from Disgaea 5, both from the same fight.
Seraphina's Prinny Squad are in hot pursuit of a single yellow prinny who made off with their sardine curry, specially made by Killia himself. Not only do they manage to corner said prinny, but they gang up on it by summoning an even larger horde. Who wins?
When the group accosts the survivor of that match, she uses Ninja Rabbit Technique: Multiply to split herself into multiple copies. The original has enough health for a single stage enemy, but the copies are punier in terms of HP. She still pulls one on the group by using Body Switcheroo with a carrot for a quick getaway.
The game includes one zone, Lake Wintergrasp, dedicated to world (i.e. not instanced) PvP. In an effort to make it more fun on servers where one faction or the other is underpopulated, it features a mechanic called Tenacity that buffs whichever side has fewer people - the greater the disparity, the stronger the buff. However, the buff was hilariously weak as it did nothing against CC effects, so if 3 or 4 people with 20 stacks of tenacity (full power, 500% EVERYTHING) encountered the other faction's main group... they got obliterated very easily, averting this trope.
And in other cases of World PvP, the side that brought 80 people destroys the organized group of 5 or 6, because players are all fairly equal, so Wo W actually averts this trope pretty hard... until you go into PvE, where it's in full effect.
Played hilariously and probably accidentally post-Cataclysm, where mobs and Non Player Characters will be found fighting each other in perpetual battle. It's entirely possible to find and especially to set up scenarios where one NPC or mob is fighting dozens of mobs or NPCs, with that one person fighting on equal footing to the entire small army that's descended upon them.
In The Witcher, one of the three fighting styles Geralt is trained in has been specifically tailored to allow him to engage a horde of up to nine enemies at once. If you invest an equal amount of points in all fighting styles, it is possible that some enemies, such as cemetaurs, will be more of a challenge to bring down on their own with the 'Strong' style than a group of them would be if engaged with the Group style.
Avoided in The Witcher 3: Wild Hunt When the Wild Hunt goes up against the witchers of Kaer Morhen and Yennefer, Cirilla and(depending on your decisions)some all or none of the following: Triss Merigold, Kiera Mertz, Zoltan Chivay, Vernon Roche, Ves, Letho, Hjalmar an Craite, Vigi, Folan, and Ermion, all with financial backing from Sigmund dijkstra. They lose, pretty hard, in fact they only survived thanks to Ciri cracking under the pressure of her imminent capture by the Wild Hunt after Vesemir's trope/Heroic Sacrifice And they only survive that because an elven sage was there to stop Ciri from becoming a human black hole. Sure they fought them off barely But in the process they lost Kaer Morhen and one of their most knowledgeable and experienced people Vesemir And the bad guys lost around 50 grunts out of the Hundreds they have. And most of the people who were on defense were either superhuman, magic, or both.
Default unit sizes are subject to this rule, the extreme being the sword master that's just one guy that can take on dozens of lesser men.
The game also has two kinds of ninja units, on each side of the trope:
One is the individual ninja, acting as an agent. When it reaches its maximum level, it can absolutely wreak havoc on the enemy by slaughtering its generals, disrupting its armies, spying its lands and sabotaging its buildings.
The other is a platoon of ninjas that can be deployed on the battlefield. While they're certainly sneakier than any other average unit, they have no armor and are worse than other well-equipped units such as cavalry or samurais. Also, they're horrible individually.
In all the Total War series a large army can be hard to properly control. Most notable in Medival II when a single unit of properly micromanaged heavy cavalry can devastate thousands of peasants. It is just too hard for a human opponent to reorganise their line to take the charge properly.
Homeworld 2 has an interesting variation where it's the heroes that have the disadvantage in numbers. When the Hiigarans show up to claim an ancient Precursor artifact, they come across a guardian boss known as a Keeper. The Keeper is a strange looking medium-sized warship which sets off to engage the entire Hiigaran fleet solo. You know you're in serious trouble when you're commanding a massive armada to fight a single enemy unit.
Trainers with a five or six-member party are usually Bug Catchers or Fishermen, and will use lots of lower-leveled Pokémon, or weaker Pokémon in general (like Caterpie and Magikarp). A trainer with only one Pokémon will be substantially higher-leveled.
Pokemon X and Y introduces horde encounters, where the player may encounter a group of five Pokémon instead of just one. However, these horde Pokémon are always at around half the level of any other Pokémon in the area.
In AdventureQuest, there is an enemy which is composed of 100 (previously 1000) ninjas. There is also a ninja enemy named Shadow Mistress Elizabeth. Guess which is the tougher to beat.
The Japanese are given the ability "Bushido", which lets their military units continue fighting at full strength even after taking damage. While not making them stronger, per se, it does mean that a single, wounded samurai is just as deadly as an entire group of them. Although there is a civic that makes wounded units do more damage.
The Ethiopians have a special combat bonus which is applied if they go to war with a faction that has more cities than they do.
Encouraged for the Indians, whose ability, Population Growth, causes them to take double the unhappiness penalty for number of cities but halves the penalty for overpopulation; in other words, their empire will be more efficient if they focus on maintaining a small number of densely populated cities rather than a large number of small ones.
In general, this is referred to as the debate of "long or tall" - building a lot of cities versus building only a few and focusing on them. Both having lots of cities and having too many people in a city will increase Unhappiness, and a number of strategies favor one or the other. Founding new cities will also greatly increase the cost of social policies, and require a lot more micromanagement. Notably, the Tradition and Liberty social policies are designed to specialize in one or the other - Tradition mostly gives bonuses to controlling a small number of cities, while Liberty gives bonuses to quickly settling a large number of cities.
The Grand Finale of Dragon Age: Origins has the capital city of Ferelden attacked by Darkspawn. When you get inside the city, you'll find that you're vastly outnumbered, but are only fighting against grunt versions of the normal darkspawn, meaning that they go down in one or two hits to balance things out. Although there areElite Mooks, mixed in with them. If you're trying to kill the Darkspawn General based in the alienage, you will have to fight through a Zerg Rush of Elite Mooks, at least on higher difficulties.
Vindictus probably has the most realistic use of this trope. Enemies use a variety of tactics, including mass mob attack, and single flanking and circling attacks. When enemies do attack in mobs, they predictably cause a lot of collateral damage, getting in each other's way as one would normally expect. This is even true for multiple boss missions. In fact, on the Gnoll Chieftain mission, the standard tactic is to keep running from the boss for the first few minutes of the fight, to allow his wild attacks to take out all his mooks before fighting him.
Can happen to the enemies in the Final Fantasy series. Certain regular enemies will resort to much more dangerous attacks if they are alone, which means that you should take them out first if they're in a group.
In the Fire Emblem games, your army is traditionally outnumbered 2:1 most of the time, though sometimes as bad as 5:1, or more. However, most of these enemies, if the weapon triangles are utilized, are hilarious pushovers. But if you get into a room in a castle mission with only one guy sitting on a throne, perhaps with a Swordmaster, General, or Bishop at his side, get ready for a hell of a fight.
Due to the limitations of the hardware, as the number of Space Invaders left in the game decreases their speed of attack increases.
Parodied in League of Legends, where the four ninjas (Kennen, Shen, Akali, and Zed) all have a hidden passive which reduces their maximum hp by 1 point for every other ninja, resulting in a possible reduction of -3 hp. The effect is unnoticeable upon gameplay, and acts as an homage to the trope. They originally had a hidden passive where each "Flippin' Ninja" (the actual passive effect) in Rift resulted in a loss of 1 Attack Damage in a more direct homage to the trope. However, after seeing that this had a very noticeable effect on the outcome of games, it was changed to the current.
Excerpt from the novelization: It quickly became apparent that the first to rush in were the wildest and weakest both. In their eagerness to do battle, they didn't stop to plan their strategies. What they possessed in speed, they lacked in forethought. He was armed and they were not, so for being headstrong beyond all reason these brutish beings paid the ultimate price.
In Company of Heroes the MG-42 gets bonuses to accuracy and suppression when facing larger number of troops.
In Spiral Knights, the more members there are in your squad, the harder enemies and bosses become. It's a well-implemented method of balancing the co-op with the soloing, but it can also mean that sometimes soloing is easier than playing in a group. Sometimes.
Kingdom of Loathing's Ragamuffin Imp familiar is a variation; it grows progressively weaker the more players are using it as their active familiar at any given time. Originally, its strength was inversely proportional to how many people had it, and when the puzzle required to get it was spoiled it quickly became useless as a bunch of people got one (1 point of hot damage, anyone?). Eventually this was changed to the present formula.
Guild Wars invokes this with a PvP effect called "Inverse Ninja Law." A player does less damage for each nearby ally, and more for each nearby enemy.
Simulating this effect in an MMORPG was a major design goal in Star Wars: The Old Republic. In order to get the most out of being one of the universe's signature badasses the game tends to pit you against mobs of three to five enemies who die quickly, making you feel as strong as a jedi/sith should. When it's down to two or one enemies though, it usually means you're fighting an elite or boss mob, making the fight much harder.
The Naga faction in Might and Magic: Heroes VI has a racial ability called honor which gives them a defensive boost. The honor gauge charges whenever an enemy stack is attacked that hasn't been attacked yet that turn; if you want to use the honor ability efficiently, you need to attack enemies one-on-one instead of sending all your troops to gang up on a single stack.
Asura's Wrath is a huge example of this trope. Whole Shinkoku and Ghoma armies? Harmless. Asura even takes an army of Ghoma after losing his arms and using a broken sword. Single bosses like the Seven Deities or Vlitra? That's trouble.
No More Heroes is a case of this with Boss Dissonance. The game's generic enemies don't pose much danger, the main challenge comes from the game's bosses.
Dynasty Warriors is probably the most (in)famous example in gaming of this trope. Players will be attacked by hordes of mooks so pathetic they might as well not even attack while the player chops them down. Conversely, enemy officers tend to take more swings to defeat - especially 'legendary' officers like Zhang Fei and Lu Bu and, in later games, the enemy army's commanders themselves.
Later games in the series subvert this somewhat on higher difficulties, as improved AI and increased stats have mooks coordinate their strikes with each other. Prepare to get stunlocked from all directions by fairly strong spear pokes that quickly add up if you're not careful.
Hyrule Warriors, being a CrossoverSpin-Off with Dynasty Warriors, naturally plays this straight. The very numerous Bokoblins go down in droves, whereas the more infrequent Lizalfos and Dodongos take longer to bring down.
A memorable moment from the original Shenmue has Ryo and a partner fighting off seventy attackers and succeeding.
The iOS game Final Fantasy All The Bravest inverts this - you have an army of up to 40 warriors of varying classes (and heroes from different games) up against a handful of monsters per round. The monsters and bosses have a load of HP, your characters are One Hit Point Wonders.
Discussed in Pirate101 by El Toro while the player invades a ninja fortress. Also played straight game play with some fights when the player is greatly outnumbered enemies are much lower leveled while enemies that fight alone are strong enough and have enough tricks to wipe out the entire player's party.
Because of how turns work in Worms, when a team of worms starts to dwindle down, they "move faster" than they did before. The game always ensures that players alternate turns, and because there's fewer worms on the team to cycle through, they get to act more frequently. At the extreme end of 1 vs 8, the 1 worm is basically moving eight times as fast as all the other worms. This helps narrow the advantage the 8 might have in such a situation, making spectacular (and usually hilarious) comebacks more likely.
Played with in Resident Evil 4: at first 5 or 6 villagers can be a pain to deal with but as their numbers increase so to do the upgrades the merchant sells allowing you to mow through them with ease. You can still be overwhelmed by their numbers if you're not careful and elite mooks like Dr. Salvidor, the Reginerators, the Garradors or the El Gigante are dangerous as hell on their own and when backed up by normal mooks they become a serious threat. When another elite mook backs them up they can easily over power you because the tactics you use backfire against two such enemies.
In the Monster Hunter games, if a mission specifically tasks you with killing multiple monsters, they will be weaker than they are in hunts where you only have to take down one, in order to keep the quest beatable within the time limit. But if another monster just happens to show up during a hunt (even if their appearance is scripted), then they'll be at full strength.
A dwarf in Dwarf Fortress who is outnumbered will enter a martial trance, gaining huge bonuses to hit, block and parry. Even civilian dwarves will do this, and then charge the invaders with surprising effectiveness.
In KickBeat, your character stands in the center of a circle while mooks swarm around you, attacking to the beat of the level's music.
Skullgirls takes this into effect with its "ratio" system: Either player can pick a team consisting of 1 to 3 characters, but the more you have, the less health each individual character gets.
Seraphic Blue has Eldritch Abominations known as Gaia Cancers that have a total amount of power that never decreases even if some of them are killed. What that means is that every time one dies, the remaining Cancers get even stronger.
In Shin Megami Tensei IV and Shin Megami Tensei IV: Apocalypse, "horde" encounters seem like they'd be able to mow your party of five (at best) effortlessly...however, the horde only gets two Press Turns instead of as many turns as there are individual enemies, shares its HP with the whole group, and all-targeting attacks will hit the horde multiple times, depleting its HP much faster than a single-target spell of the same tier can.
Diablo II: This works against the players. The more players are playing in the same game at the same time, the more powerful the monsters become — thereby making each player proportionately weaker than if he was playing on his own. With a good team setup, synergy means the players still come out ahead in that race.
In an interesting note, players can make the game think they're in a party while in Single Player thanks to console commands. While this makes the monsters tougher, it also causes them to drop more items and yield more experience. With a good build, it is entirely possible to beat the game solo in a difficulty setting meant for up to 8 players.
A variant of this appears in many different sorts of First-Person Shooter titles based on historical or modern settings, such as Battlefield or Call of Duty in regards to bullets. The fewer the bullets in the magazine, belt or what have you, the more powerful the weapon is, even if all three weapons use the same type of ammunition. While realistically speaking, the power of a bullet and the lethality of a weapon are dependent on several factors, most of these games will separate them based on a strict hierarchy, with sniper rifles having the most power, and light machine guns having the least. Guns of the same category also tend to have more or less power depending on how many bullets are in a magazine, i.e. an MP5 firing 30 rounds does more damage than a P90 firing 50. Usually, this is for reasons of balancing. A light machine gun with the same one-shot killing power as a sniper rifle would be a very overpowered weapon, so naturally, it's less powerful even if both it and the sniper rifle fire the same kind of bullet, while conversely a sniper rifle that requires four or five shots to kill an enemy player would be seen as hilariously underpowered by most.