Question

what is the effect of a larger voice box in grown up boys​

Answer 1

the voice of boys become heavy

Answer 2

It's the larynx (or voice box) that's causing all that noise. As the body goes through puberty, the larynx grows larger and thicker. It happens in both boys and girls, but the change is more evident in boys. Girls' voices only deepen by a couple of tones and the change is barely noticeable.