MT is also way more aggressive with dead weight cards thrown in your deck as you ascend. StS just has ascender's bane, while this game throws in the equivalent of 2 Curse of the Bell, 2 extra strikes, and 2 extra defends. It also gives you a lot more opportunities to remove (or at least add consume to) cards, but also often at an opportunity cost. You start with 21 cards from cov17 on, whereas you start with about 10-12 depending on character in Spire. Even ignoring units, you're still sitting at 16 cards in your base deck in MT. If you want to get rid of all your starters and stewards, you need to purge 14 cards over the course of a run, which is a ton even for this game.
tl;dr This game punishes deck bloat harshly. Spire does as well on later ascensions but this game is particularly aggressive with built-in bloat.