StS lets you not take a card when you are offered one, and often this is the best play, particularly as you go later into the game and have a deck that is stacked with powerful cards.
Fundamentally having more cards is bad, because it increases variance in what you can draw. Obviously taking a good card can be worth that cost, but often players fall into the trap of assuming the best card of the options given is worth taking, when really the question should be "does this card beat the average card in my deck, or does it dilute the power of the deck", because you are reducing the chance to draw one of the other cards in your deck.
Obviously there are edge cases: sometimes something may reduce the power of your deck right now, but introduces the chance for later synergy, which may be worth the cost—especially early on, but in general it is worth considering not taking the card.
The ideal deck is always the minimum number of cards that allow you to pull off your win state, everything else is just stopping you getting there (this is also why cards that allow you to draw cheaply are good, and one of the reasons why removing cards from your deck is powerful).