Joan Sutherland: when she lost the weight, she couldn’t seem to sustain the great sound that she had made, and the body seemed to be too frail to support that sound that she was making
Michael Scott has proposed that Callas's loss of strength and breath support was directly caused by her rapid and progressive weight-loss
Photos and videos of Callas during her heavy era show a very upright posture with the shoulders relaxed and held back. On all videos of Callas from the period after her weight loss, "we watch... the constantly sinking, depressed chest and hear the resulting deterioration".[53] This continual change in posture has been cited as visual proof of a progressive loss of breath support
In recordings from 1954 (immediately after her 80-pound weight loss) and thereafter, "not only would the instrument lose its warmth and become thin and acidulous, but the altitudinous passages would to her no longer come easily."
When asked whether he felt the weight loss affected Callas's voice, Richard Bonynge stated, "I don't feel it, I know it did. I heard her Norma in 1953, before she lost all that weight, and then again afterward, and the difference was incredible. Even more incredible was that the critics didn't write about it. When Callas was at her best vocally, she was fat, but she got only a quarter of the recognition that she got after she had become thin and was a great star."
но я конечно, в отличии от высказывавшихся (см.выше), не профессионал в данной области (как и вы, позволю себе предположить)