Koyuki's bust got absorbed by Maria in order for the latter to return to what she had at her introduction and ditch her previously awful character. The author could simply let them both be busty, but...I suppose more than one with such good looks AND appealingly affectionate character is just too much for a story like this, to him and his peers.
But there can always be a "loli": as long as the setting has a girl that looks like either Koyuki or Maria, there must be one.
That said, what's with this "Ice-chan" business? Correct me if I'm wrong, but wasn't her having Touma killed the reason he got sent to that world in the first place? Why does he want to be "friends" with his murderer? What was the purpose of his "Collapse Detector" if he, in the end, is not just okay with but thinks highly of the person who killed him through such a collapse?