I just realized it but on the last page, Shuna has sleeping-caps on her horns and Shion has a mandarin stuck on hers... so cute, such detail, much wow!
@Qelix I think it has to do with Rimuru's physical appearance when in human-form and his mannerism that gets him mistaken as a girl... Not everyone knows that Rimuru was a male in his previous life.
@Guspaz it because he still has child form as his base, so of course it still flat, while Shizu already "ara-ara" type...
later in the story after he evolve, his base form become teenager (15-17 y/o body) but still flat, since he's genderless... but a ultimate beauty...