>>149435310
She's in middle school, Sissi is 12-13, so, it makes sense that she has small breasts. That said, her chest size is inconsistent throughout the show, when she's wearing a bikini her breasts look medium or almost average, and when she's wearing her usual clothes or is only in her underwear, her chest looks small but not flat. Yumi is more consistent in that aspect, her chest size doesn't vary that much.
>>149435483
Some people probably sleep like that in real life. Sissi doesn't always sleep in just her bra and panties though.