I'm pretty sure there's no empirical basis for linking poundage with a big voice, as opposed to correlation. Crash diets are certainly not good for a singer -- any such radical messing with one's instrument needs to be approached cautiously.
(Callas' vocal difficulties may have been exacerbated by too much weight loss, but her problems lay deeper than that. I have tapes of her early in her career, and even then there were these sudden mysterious lapses.)
I've got the old tapes of the late 40s and 50s, the Mexico city and some Scala, Aida, I Puritani, Vespres Sicilianes, Nabucco, one old Tosca I don't have but that goes back too; and I never heard any lapses, although I'm not doubting you--just never heard anything that stuck out. I'd be interested to know what you mean specifically. And there is simply no comparison to what she did after 54 or 55, those of use who love those early recordings don't even hear it as a the same singer; for me anyway, other than Flagstad, there hasn't been any other opera soprano as Callas was in those early years (and to think she had a career in Greece even before those). Most of the rest of the time, as is well-known, the high notes were often of a strident ugliness that literally hurts the ear. She, of course, had 'problems' in all areas of life, I wouldn't be surprised if her life wasn't a constant fugue state. But I won't even listen to most of the later recordings anymore, the difference is too incredible.
(Apologies if I repeated things from other contributors to the thread. This one started well before I arrived, and I've only had time to get through about half the thread just now.)
dirac--sorry, I had meant 'not doubting you' originally, and didn't proofread. You may know these recordings better than I do, I just usually find them a lush vocal paradise.