Reading and comprehension, guys.
Chapter 22: an entire chapter of Shiina pondering whether she should buy a half-off cake or not. In the end she does, and she breaks the 60 kg barrier.
Chapter 23: Shiina tries to shed some calories going for a 35 km walk across Japan. However, she ends up eating a super-sized meal plus three different desserts. The narration spells this out: "the meals and desserts cancelled the effect of the walk"
Chapter 24: Kataoka sees Shiina at the gym and he notices she's been putting up some serious weight. She does not want to admit it, but in a roundabout way she asks Kataoka to help her exercise regime.
The reader: "why she fat? it's not like there has been an arc explaining exactly why..."