In the early years of her career, Callas was a heavy and full-figured woman, though she admitted to weighing "no more than 200 pounds." Despite her figure, Meneghini and others considered her beautiful, but during her initial performances in Cherubini's Medea in May 1953, Callas decided that she needed a leaner face and figure to do dramatic justice to this as well as the other roles she was undertaking:
I was getting so heavy that even my vocalizing was getting heavy. I was tiring myself, I was perspiring too much, and I was really working too hard. And I wasn't really well, as in health; I couldn't move freely. And then I was tired of playing a game, for instance playing this beautiful young woman, and I was heavy and uncomfortable to move around. In any case, it was uncomfortable and I didn't like it. So I felt now if I'm going to do things right—I've studied all my life to put things right musically, so why don't I diet and put myself into a certain condition where I'm presentable.
During 1953 and early 1954, she lost almost 80 pounds (36 kg), turning herself into what Maestro Rescigno called "possibly the most beautiful lady on the stage". Sir Rudolf Bing, who remembered Callas as being "monstrously fat" in 1951, stated that after the weight loss, Callas was an "astonishing, svelte, striking woman" who "showed none of the signs one usually finds in a fat woman who has lost weight: she looked as though she had been born to that slender and graceful figure, and had always moved with that elegance." Various rumors spread regarding her weight loss method; one had her swallowing a tapeworm, while Rome's Pantanella Mills pasta company claimed she lost weight by eating their "physiologic pasta", prompting Callas to file a lawsuit. Callas stated that she lost the weight by eating a sensible low-calorie diet of mainly salads and chicken. Some believe that the loss of body mass made it more difficult for her to support her voice, triggering the vocal strain that became apparent later in the decade (see vocal decline), while others believed the weight loss effected a newfound softness and femininity in her voice, as well as a greater confidence as a person and performer.
Callas's voice was and remains controversial; it bothered and disturbed as many as it thrilled and inspired. Walter Legge stated that Callas possessed that most essential ingredient for a great singer: an instantly recognizable voice. During "The Callas Debate", Italian critic Rodolfo Celletti stated, "The timbre of Callas's voice, considered purely as sound, was essentially ugly... yet I really believe that part of her appeal was precisely due to this fact. Why? because for all its natural lack of varnish, velvet and richness, this voice could acquire such distinctive colours and timbres as to be unforgettable." In compensation for the lack of classical beauty of sound, Callas was able to change the timbre of the voice and her vocal color and weight at will and according to the role she was singing, essentially giving each character her own individual voice.
Maestro Carlo Maria Giulini has described the appeal of Callas's voice:
It is very difficult to speak of the voice of Callas. Her voice was a very special instrument. Something happens sometimes with string instruments—violin, viola, cello—where the first moment you listen to the sound of this instrument, the first feeling is a bit strange sometimes. But after just a few minutes, when you get used to, when you become friends with this kind of sound, then the sound becomes a magical quality. This was Callas.