The origins of the phrase may be uncertain. Has anyone ever heard that Sophie Tucker is the lady behind the fat? (I'm crazy about her, although I don't know very much of her work.) Someone told me recently that in vaudeville, the best act was the last--and that Sophie Tucker was the best of the best, the vaudeville queen. This person also said that Ms. Tucker was most likely the fat lady. I'd only heard reference to opera in terms of that last best fat lady, but, hey, these things do get mixed up and assumed over time. If Sophie deserves the title, then I will carry forth her promotion for whatever trifling effect I can make.

Sunday, Sept. 19, edit: wwh checked and Quinion writes that it was opera. I think I might write to Quinion and ask whether he's heard the Sophie Tucker theory...

Thanks, wwh, for checking!