There was a time in the opera world where singers believed that they had to be fat in order to have a big sound. Also, the role of Brunhilde in Wagner's opera was supposed to be a big Viking lady who was fat. So, many female singers believed that they needed to be large in order to get the role. But I think that it mostly had to do with the opera being almost exclusively a stage production with almost no film and TV. There was never any pressure on opera singers to have the same bodies as film and TV stars because they always performed on stage and often wore big bulky costumes (save and except for Strauss' Salome). There is also a theory that opera singers would perform on an empty stomach because it is very difficult to sing when right after a meal. Then, they would go out and chow down at a fancy restaurant after a show or long rehearsal day.
Today, opera singers no longer believe that weight helps their sound. In fact, many are very focused on fitness and wellness as the top singers are very much in demand and work a very grueling schedule.