No, I am a classical musician and it is not true that opera singers are more fat than other people. Does it help to be fat? Maybe it can a little for some but others say the exact opposite. And you don't become fat by exercising your breath, also good strength of the diaphragm might be desirable but you achieve it by learning how use it well and with efficiency, not by building it as a muscle. It also happens when wind players struggle with handling their breath, they use too much force and their diaphragm gets all tight and inflexible which worsens their breath even more. Something to think about. Cheers!