I thought I would quickly add this tidbit of information. When I recorded the aria from Tosca on my page I was 16 years old about 5'9" and roughly 120-125 pounds, size 26-inch waist. I'm about 5' 11 1/2 tall now and around 170. The reason I mention this is many of the people I've worked with and have been around in Opera tend to reach unhealthy sizes and have an extreme fear of losing the size and capacity of their instrument if they lose weight. As such early on I found they try to fatten you up.
Here's some things to remember:
1. There are many small people with huge voices- there are many large people with smaller voices. I would suggest proper breathing and muscular development. Instead of packing on excess baggage which will ultimately cause your heart to have to work harder.
2. For those large ( overweight) crash diets may cause you to lose your voice, vocal control as your body physiology changes, however if you want to lose weight without the negative effects simply slowly modify your diet and hit up cardio. You should think long term not quick fix. In this manner you should not see any loss of range, power or dynamics. You may actually see an increase.
3. Remember voices can gain unhealthy weight as well, this will make the passagio difficult and cause constriction. You should have a full range of usable, colors, weights, textures and shadings within healthy parameters. these parameters grow naturally in proper development and usage and should not be forced, rather slowly and carefully stretched. I find it is always best to leave an exercise knowing you could have taken it up or down an additional step then to push it. Your confidence will increase more because you won't feel like your struggling for the notes. The voice will set up and you will find it of it's own will guide you to next higher or lower step, because it will become so easy and the register transitions will occur almost un-noticed.