So you're Hiroto, huh? You've been strutting around acting big all this time, haven't you?
Literal
You [subject-が] Hiroto [question-か]? Until-now a-lot big-face do-[for-me-favor-てくれた] [ね].
おめー is a rough, aggressive masculine contraction of お前 — menacing. でかい顔する is a fixed idiom meaning 'to act like a big shot, to throw one's weight around' — literally 'make a big face.' The use of ~てくれた here is sarcastic: くれる normally marks a favor done to the speaker ('you did me the favor of'), so the speaker is mock-thanking the other for having acted big — a classic Japanese sarcastic inversion. The drawled final なあ extends the sneering intonation.