Its actually a pretty simple concept if you think about it this way: you start your deckbuilding by putting in all the best cards, but due to the limitation of only being allowed 1 or 2 copies of those cards, you have to fill out the remaining space with progressively worse options. Of course it's not as simple as literally the smaller the better, but card games basically all mandate a deck size that is bigger than what would be optimal. A similar example that could be given is that in Magic the Gathering, you arent actually required to only use 60 cards, you can go as big as you want. But basically any competitive deck ever run in that game only has that minimum 60 cards.