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I think the design process went like this.
Cover assigned the basics: "we need a tall girl, short girl, beautiful girl....etc"
The girls decided among themselves which one they wanted to be.
Then they assign the themes: "We need time, space, civilization....etc"
The girls decide among themselves, probably a lot of bickering.
Then Cover works with their talents fill out their character traits.
"Short girl, you be the excitable kouhai. Tall girl, you can be the femdom girl."
Then the characters are designed with some basic constraints in mind, but the girls get lots of feedback into their designs. All the girls want to be sexy, so they all request the biggest boobs they can get away with. Cover needs variety, so they don't let everyone have massive boobs. The taller characters have more leeway to be sexy, because they are less "cute" by japanese standards.
It makes these designs far more sexy when you realize the girls themselves probably chose these designs to titillate you