Context: Nishikata 西片 remembers something he heard from someone who heard it from someone else.
{mune no chiisai} joshi tte,
{sono koto wo
ki-ni-shiteru} baai ga
ooi rashii ze. 胸の小さい女子って、
そのコトを
気にしてる場合が
多いらしいぜ。
Speaking of girls [with] {small chests},
I heard that,
the cases [in which they] {be worrying about that are many}.
i.e. there's a rumor flatter-chested girls often have complexes about it.
ki ni suru 気にする
To mind. To think about. To care about. To worry about.
{mune no chiisai} joshi is a relativized possessive double-subject construction with a no の subject marker.
tte, san-kumi no
Kimura ga...!! って、３組の
木村さんが…！！
Is what Kimura from the class 3 said!!
Kimura is marked as subject (Kimura ga) of a sentence that has no verb.
Because of the quoting particle tte, we can safely assume the implicit verb is supposed to be "to say," iu 言う.
