It is not true that heavier/fat people have a great voice because they have large resonance.
The resonance size does not increase – even if you gain weight!
Sound produced vibrates in the air and it is required to have enough space to put/store the air, which is the resonance chamber.
The resonance chamber is composed of1:laryngeal ventricle 2: piriform recess 3:pharyngeal cavity 4:oral cavity 5:nasal cavity.
You have to make the above space big enough to resonate the voice.
I think that many obese people have a small resonance chamber. This is because the amount of tissue also increases, with the larger statute of the body, to cause a loss of space in the throat. However, lung capacity will be increased and is generally in proprtion to the body per hereditary traits.
If the body’s outside was big but the inside (similar to an instrument) was hollow, then the voice would sound good … but it’s not scientifically possible for a human. Obesity does not necessarily mean a greater sound or sound capacity, people of any size can have a good voice.