、。
You used to be flat-chested, and now somehow you've gotten this big — damn it.
Literal
Long-ago [topic-は] flat-chested was [although-のに], before-one-knew this-much big become-[contempt-やがって].
A crude, masculine, intimate register. ペチャパイ is slang for 'flat-chested' — a blend of ぺちゃんこ ('flat') and パイ (clipped おっぱい). ~やがる is the pejorative auxiliary expressing disdain, annoyance, or rough familiarity toward the subject of the verb — it adds a 'damn it' flavor. The sentence trails off with ~やがって and no main clause — a common 'leaving the complaint to hang' style. のに marks contrast ('even though').