I always judge an ass when the girl is standing, that's why I posted the pics above. Because girls can easily cheat by bending or arching their back (and they do it often).
Maybe you like thicker girls so I can see why you would find Sora too slim and/or not curvy enough.
(It was difficult to get the exact same position)
You can see the fat on the left, not on Sora's. It's the main difference and the shape is also not the same (more square than round).
Surprisingly, Sumire Kurokawa is rarely mentioned while I see Yuu Shinoda's name over and over again on Reddit or forums. Yuu is flat now. When she's standing, her ass is flat (she lost weight over the years). I prefer Sora over Sumire as her face is prettier.