I work in a bariatric unit-(gastric bypass patients). There's a before and an after procedure for every single body system in relationship with the aftermath. There's a lot of gain, but substantial losses too, particularly to the gastrointestinal system. There is basically a complete makeover to the abdominal walls, being the most common danger what we call "dumping syndrome".
That makes sense, and opera singers use their diaphragm and stomach......so the technique has to be relearned, because skinny singers probably use muscles and heavy singers use a combination of muscle and weight (the weight helps push more air up and out). So without the extra weight, suddenly you need more muscles controlling the diaphragm. I don't know the exact mechanics, but I have had a voice teacher explain this. I think someone who learns to sing while thin and gains weight and then loses it knows how to adjust, but if you have been heavy your entire life and learned to sing while heavy, I think it is a big adjustment to relearn your technique, and some voice teachers are scared to help a singer re-learn his/her technique, because people develop habits (both good and bad) and re-teaching how they use their body could totally ruin the voice.