Zoku Owarimonogatari - Episode 2 (Patreon)

Fried once again lol.

Did some more digging.

So this "mirror world" doesn't mirror people physically per se, it reverses their characters. That's why Kanbaru and Hanekawa are their oddity forms, because those forms are a small fraction of their actual personalities in the real world. Tsukihi basically didn't change, because she was already 100% comfortable with herself from the jump.

The world reveals "hidden other selves" that otherwise would have been overtaken by the regular personalities we've seen. Mirror Sengoku is assertive because the real Sengoku had a secret assertive side, as we've seen in the Hitagi End arc. Sodachi is staying with Araragi and is happier because part of her character in the real world wished she stayed. Mirror Karen is smaller because part of her didn't like being tall. Mirror Hachikuji was an adult because the real Hachikuji wanted to be one.

Because of this world's nature, Araragi is able to see the entirety of his friends' personalities, the hidden sides they don't want others to know about.