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The opera isn't over till the fat lady sings.
Literal
Fatten-[past-adnominal-た] woman [subject-が] sing [until-まで] opera [topic-は] not-end.
A direct calque of the English idiom 'it ain't over till the fat lady sings' (don't declare defeat prematurely). The idiom isn't native to Japanese, so this sentence feels like a translation exercise rather than natural Japanese. Structurally it's a clean example of ~まで 'until' taking a verb clause as its complement, and 太った (past/adnominal of 太る) functions adjectivally as 'fat' — Japanese has no 'to be fat' adjective, so it uses the resulting state of 'having gained weight.'