There's a simple physical explanation, but the causation is the opposite way round from the way people usually think.

When a good singer produces clear, focused tones, each lung radiates spherically. Over a period of time, this vibration migrates the subcutaneous tissues into a pair of symmetrical, globular forms. Meanwhile the lower partials associated with the diaphragm are channelled downwards and reflected by the pelvic bones to induce the development of similar forms lower down, on a larger scale because of the lower frequencies.