They see a bodybuilder wearing a tight fitting muscle shirt that shows off his great build. They decide his genitals should match his muscles and it begins to swell.
His work out shorts begin to round out as his balls grow thicker and heavier. His dick lengthens down his leg and stops at a monster nine inches. His balls stop at about softball size. His shorts are clinging to him and threaten to expose his now huge package.
The boys see their next lucky test subject.