I've been running 30 convoy decks for a long long time now. It basically happened when the # of wounds before death was changed to 1 (which was ages ago). Before that, I could run a totally static deck (the same cards could just stay in forever, and get healed at town). With a static deck I really tried to optimize it, and ran relatively small decks.
Nowadays bigger decks are just better. And you make substitutions after almost every single fight. There is no point in optimizing a follower deck really hard, since you will just switch out everything that got wounded.
Bigger decks are also good for other reasons:
1. You can cycle convoy cards pretty much all you want, looking for followers that better fit the current amount of momentum.
2. You will always be able to run the enemy out of cards before you run out of cards. (pretty much biggest reason to run 30 cards)
There are also many followers that I value about the same amount. Those all get subbed in for each other. If i value all my followers nearly the same amount, then I might as well put in 30. There are a couple followers that are a bit stronger than others, but not enough to really worry about. I have no special "need" to draw these followers very fast.
If i don't have really important, very powerful followers, then I don't need the deck to be small. Every one of my 30 followers is "good". None of them are super amazing. I don't need to make sure i draw my "power cards" quickly. I just need to make sure i draw a follower at the start with 3 momentum cost.