I..
Dont know what to say
oh wait Eirin face tells it

very small...

I still don't get it. Is it that her ass is really small or what?

At the risk of using tropes on a month old comment, I believe the reasoning is a combination of "innocent fanservice girl" and "even the girls want her".

I still like the look on Yuuka's face, a look that says "Not this again..."

Why do Kazami Yuuka's and Yasaka Kanako's gym shirts have their personal names on the name tags while Yagokoro Eirin's has her surname?