The vocal folds start to weaken eventually because of age, and sopranos suffer the most. During a tenor's high C, his vocal folds close 500 times a second; for a soprano's high C, the rate is about 1,300 times each second. When menopause hits, the loss of estrogen lowers women's voices and they lose their highest notes. Since fat cells produce estrogen, though, obese opera sopranos tend to be more resilient. They can keep on singing.
--Michelle Tsai, Slate, on why we see the fat lady singing