The only way that I know of that weight has to do with singing is about endurance. If you are going to sing a 3 hour opera, you had better have some extra weight on you at the start, because you are going to lose multiple kilos of weight during your performance. Just like boxers and other athletes. The kind of singing that is done in opera is very, very athletic.
With opera singers specifically, there is a tradition of rich food and wine in opera culture, just like there is a tradition of marijuana in the cultures of some other musical styles. And I don’t think the opera audience or critics judge an opera singer based on their weight, which is very contrary to singers in other musical styles. In opera, it is all about bringing the goods vocally. In other styles, it can sometimes be a modeling show where some singing also goes on. So generally speaking, opera singers may be a little heavier than other singers.
Regarding the physics of the voice, you can be a very small person and have a large vocal instrument, or a very large person and have a small vocal instrument. The size of your sinus cavity has a lot to do with the resonance and size of your voice, and there are very small people with big noses and large people with small noses. When you put all the variables together, I don’t think you can make any assumptions about physical size and especially not about gender. If you were holding auditions and a tiny woman walks in, you should definitely not be surprised if she blows your hair back with her vocal power. I have seen that again and again in my own experience. Also don’t be surprised if a giant man comes in and has no power at all. Also very common.
A big part of a powerful voice is training. If you learn how to sing powerfully from the diaphragm with a relaxed and resonant vocal instrument, you will generate your maximum power. And generally speaking, that will be much more power than an untrained singer. (Keep in mind, though, some people train at church or at home — you don’t have to have gone to college to be trained.) Learning how to project the voice is a big part of generating volume and power, and the instant identifier of an untrained singer is they can’t project the voice. And then once you are trained, you have to practice regularly to keep everything in shape, just like an athlete.
So the way to judge if a person has a powerful voice is to just to hear them sing.