Another moment Minami feeling uneasy about her chest, like following
Yutaka: You'll look nice in yukata!
Minami: ...I know you are being nice, but somehow I can't heartily enjoy it.
It's just like... you're saying I don't have...
Behind this conversation, Minami had in her mind a rather common assumption that people with larger breasts doesn't fit in the kimonos very well, being easier to look fat and tubby.
(And probably Yutaka had no idea about it, or simply no idea about Minami's source of grief itself.)
(Superfluous comment: I thought it goes without saying if you are a Japanese, but it looks like many failed to catch the idea, so for that reason.)