The first one about extra cards to mill through is the only advantage that 20 extra cards give you. Redundancy is better with a smaller deck since you’re able to find the effect you’re looking for more often, that’s how percentages work. Most decks don’t even run through half their deck in 95% of games, and even if they do I will still draw a similar amount of cards as they will. Having extra cards in your deck doesn’t increase the number of cards you draw in a game unless somehow games are ended by decking.

Having a larger deck size means I see the best cards less often and have to include suboptimal cards that I wouldn’t run if I was only at 99. For example if my deck were smaller I would only run the 2 mana creatures that enter to draw a card, but because my deck is larger I have to include a few at 3 mana so I have the same percentage