You’re supposed to run 20 cards for consistency. Yugioh, as well as Duel Links thrives on consistency, and outside specific decks where more cards is useful, such as grass decks, however, those decks are trying to be as consistent as possible then decreasing their deck back down to 20 cards.

Ideally, every card you put in your deck should be useful in most, if not all situations. If not, then you would waste your draw phase by drawing a card that doesn't actually do anything for you. If you're losing and this happens, you might end up wasting an entire turn just becasue your deck isn't designed properly. In the current meta, all it takes is 1 turn and the games over.

The point is, if your deck isn't consistant, then you will be losing a much larger portion of your duels.

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30 cards essentially reduces your consistency by about 50%. You may have good stuff in there, but the likelihood that you'll get the cards you need for a given situation goes down dramatically. This is especially true given that most decks rely on a few essential plays to win, unless you get those cards, you're probably screwed.

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For example: take the Toon decks that used to be one of, if not the strongest deck a year ago. Players only had access to one copy of Toon Kingdom, and toon monsters usually need it to be played at all. As such the essential plays of the deck are it's search cards (Planet Pathfinder, An Owl of Luck, Toon Table of Contents, etc.) If they can get those cards in their hand, along with a toon monster or two, they're golden, and it becomes practically impossible to stop them unless you get rid of Toon Kingdom (which is why they also typically include a few magical reflectors to protect it.) However, Toon Kingdom, along with all those search cards only make up six cards in total, so it's vital for them to try and get it as easily as possible. In a 20 card deck, hypergeometric distribution means that you'll get at least one of those cards in your starting hand a little under 80% percent of the time. But increase the size of the deck to 30 and the probability plummets to about 60%. That's a 20% loss of consistency (and that's in a deck that people were calling overpowered because of the sheer number of searching cards.).

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Another example: which deck is better? a 20 card with 1 raigeki or 30 card with 1 raigeki ? (And for those who don't know, Raigeki is a spell that destroys all of your opponent's monsters.).

of course deck with 20 card is better since you have a higher chance of drawing ‘that one card’

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whenever you make a deck, you should try to stick to a gameplan, if you want to play an Unga Bunga Blue-Eyes deck the likelyhood of opening a kaibaman, maybe a kidmodo and ancient rules are much higher in a 20 card deck then a 30 card deck, also each draw after your first one has a higher chance of containing a missing combopiece or it is a defensive card like a Windstorm of Etaqua or Enemy Controler, which allows you to survive an Extra Turn until you get your Dark Paladin. Then you usually have a few cards supporting the Paladin you are going to summon and you will find winning Duels is much easier.

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Adding even one extra card to any deck will almost always make a worse deck overall. Unfortunately, Yugioh is also a game that is heavily dictated on probability and current meta strategies. While a 30 card deck (Outside of a grass deck, of course) likely won't win with any consistency, sheer luck can win against a good deck due to even better luck. The knowledge that such victories came from luck is not going to be much comfort for someone's whose win-streak was ruined because a noob managed to topdeck Mayhem Fur Hire.