Sexually speaking, there is a myth suggesting that the size doesn't matter, it's what you do with it...?
I highly disagree with such notion.
Size matters always and in all instances.
Let's say your shoe size is 8, but you are offered a shoe of size 10 or size 5. Would you be able to wear it with comfort?
I personally don't think so.
We are all different sizes, weights and heights and respectively, our internal and external organs are different in sizes and appearances as well.
The sound produced by the singer very much so depends on his height, weight, and the actual circumference of his/her body.
For example, if the person is tall and slender, usually that person would have longer and thinner vocal cords.
Therefore, there is a myth that the opera singers with big and rich voices, are naturally big and fat.
Big in sizes, maybe, but being fat, not necessarily, and actually can be counter productive.
In any case, the human being has to be physically, emotionally and in our case, vocally fit.
But yes, the body size and the size of its organs definitely matters.
If you return back to the slender and tall person, we can pretty much assume that vertically, the sound will be soaring straight to the Universe, but that singer might be lacking the width and the body of his/her sound, simply because his/her body's rib cage is quite narrow an thus, does not produce the needed radius to make the sound proportionally rounded.
Is there a solution to this problem?
Yes there is.
The person could exercise their upper diaphragm by opening it and closing it, while concurrently counting as long as they can.
If you visualize an accordion, which works on air, that probably will resemble the same action.
The singer also could consult the private fitness trainer to get a suggestion on certain exercises to enlarge the size of the ribcage and strengthen the upper diaphragm and the supporting back muscles overall.
Let's look at the person who is wide enough, but not tall enough.
That person has a great diameter of his/her vocal cords, but most likely, does not have the length.
How would we solve this problem?
It's much harder to work on the extension of the height from the physical point of view.
But what the singer can work on is on the higher, so to speak, "bite", involving the strength and depth of the facial muscles.
This type of singer also has to pay greater attention to the support coming from, not only his/her lower abdominal muscles, but also restructuring the whole weight, starting from the feet, to the calves, to the thighs, to the lower abdomen, to the upper diaphragm and shoot the sound on top of all of that.
That will greatly increase the needed length and then in conjunction with naturally given width, the sound will be fully projected, aimed and delivered.
So that said, the size does matter and very much so in every instance of our lives!