Weenie decks are something we see in both Yugioh and Magic, with both being considerable decks in any style of the game itself. So what are weenie decks? Basically, weenie decks consist of a deck that relies on smaller fighters to gain victory and overpower your opponent. So let's take a look at Weenie Decks for both Magic and Yugioh.
Magic Weenie Decks:
In MTG, weenie decks consist of decks that function well with everything being of a cost of 3 mana or less. This includes creatures, auras, equipments, and instants that combine to make the deck a force to be reckoned with. In weenie decks, most creatures would cost between 1 and 2 mana, with creatures of 3 mana still seen but in lesser numbers. Creatures with converted mana cost 4 or more normally tend to be the "boss monsters" of the deck, usually having some kind of relation to the deck. In times, weenie decks may use Planeswalkers as their boss monsters as can be the case of Elspeth.
Elspeth not only can create 1/1 soldiers, but make your smaller creatures a lot more powerful. There are other planeswalkers that follow their own distinct strategy, but some such as Garruk (the new one), Gideon Jura (with his final ability), and Ajani Vengeant can all be good considerable bosses for weenie decks.
But weenie decks aren't all low cost creatures, they also depend on stuff to turn them into considerable opponents. Whether it be Auras or Equipments, these cards can easily turn your 2/1 weenie with flying (random example) into a 4/3 flying with double strike, for example. Cards like these provide easier control of the game, as you can not only gain bigger creatures for a lower cost, but keep mana reserved for any defensive spells you might play.
As a final verdict, MTG weenie decks rely on a combination of low cost cards to overpower your opponent and swarm the field with an army of small creatures that can do big damage. Weenie decks are known for gaining big advantage in early games, so they can be a force to be reckoned with.
Yugioh Weenie Decks:
Now, while YGO doesn't have any resource system, it can still have weenie decks. These, however follow a similar concept of using small monsters to gain big advantage, but weenie decks here have changed across the ages. Most weenie decks now have the ability to Synchro summon massively, but since the bosses come from the Extra Deck, the main deck can still consist of weenies to make the deck work. However, there are still decks that are true to the weenie deck nature, while there are others in similar nature but use their weenies for a bigger purpose.
The first weenie decks we see in YGO are level 3 or lower monsters that are the core of the deck, and have a strategy that revolves around them. An example of a pure weenie deck in YGO can be a Wetlands oriented deck. All their monsters are level 2 or lower and use the field to gain a major buff that allows them to compete with the big monsters on their own. Not only that, but it gives them access to bigger monsters with greater ease. For most cases, these weenie decks use either fields or continuous effects (from either spells, traps, or the monsters themselves) to gain attack, while mostly ignoring equipments (contrary to MTG where auras and equipments buff individual monsters).
And then there's the other type of weenie deck, which uses a combination of level 3 or lower monsters to gain access to bigger monsters (such as Synchros or Xyz). Decks like these consist of massive swarming with said creatures and bring out either 1 or 2 big monsters by either using their effects to recruit from the deck or be used in conjunction with other monsters to gain advantage. Weenie cards that fit this category are monsters like The Immortal Bushi, Deepsea Diva, Junk Synchron, and Tour Guide From the Underworld.
Said monsters can for bring their own massive plays. Junk Synchron can be summoned if Bushi is on the field to bring back a level 2 and make a Junk Destroyer, one of the boss monsters of their deck. Diva recruits for easy level 5 synchros on her own, or combined with another Diva can make 2 synchros if combined with other monsters. As for Tour Guide, it can make an Xyz monster out of thin air due to it's effect.
While MTG Weenie Decks may be of a similar fashion, they all tend to be pretty powerful on their own. Meanwhile, YGO Weenie Decks tend to have multiple purposes. While one of their weenie styles may be similar to the MTG mentality of small creatures gaining huge power, it's the "weenie swarm for big bosses" strategy that is predominant in this game.
Weenie decks can be a force to be reckoned with if not careful. Just because you see small monsters, doesn't mean they are easy to deal with. For these decks, there's strength in numbers.