About the three mail girls, are their bodies supposed to be portrayed like this:
Rin: slightly flabby butt
Mimi: big boobs
Kuro: all around flat
In the manga, they're all very young, but later on they become teenagers - that's when the above change happens.
OP image is also from when they're teenagers and not kids. You may remember a short episode that was included in the very last volume (the one after vol 13) where Rin's slightly flabby butt gets stuck in a small hole in a swimming pool. The other girls pass through, as if Watashiya wanted to show that Rin's butt was different.
Kodomo no Jikan is no stranger to handling sensitive subjects. Do you think Watashiya - on purpose - made Rin's butt a little flabby as a teenager? I said a little. That means no cottage cheese cellulite, but squishy butt flab that the other girls' butts appearently lack because they're a lot firmer.
http://www.strawpoll.me/12363154
It wasn't just post production, her butt was portrayed as flabby in the manga as well.
>>153607020
It was? I never noticed that. Which volume?