Under the tissue paper, you discover a sort of skimpy speedo-type thing. You realized that it's the type of posing trunks that bodybuilders wear on stage. The trunks are black and shiny, like satin, but made of stretchy spandex.
"What an odd gift!" you think, as you turn the trunks over in your hands. You won't be wearing them of course. You'd feel silly. And the front pouch is so big! You can't imagine what kind of man would fill such a roomy pouch.
Almost without thinking, you remove your clothing, still not intending to put the trunks on. You're not a bodybuilder, after all. Even as you stand in your bathroom, completely nude, you don't intend to put the trunks on. So it's quite a shock when you pull them up your legs, around your waist.
For something so skimpy, the trunks sure are big on you. They hang loosely on your hips, and the front pouch is tragically unfilled. Even as you wonder how it was that you put the trunks on, you feel a strange tingle run throughout your body. The tingle intensifies, as a fiery pain engulfs your entire nervous system, each nerve ending screaming in agony.
As you fall to the floor, you notice that your arms seem to be swelling and thickening. Veins pop to the surface, as hard muscle appears. You slowly rise, the pain subsiding into a dull ache. In the mirror, you watch a built young stud grow larger, muscles bulging with power. Hesitantly, you lift your arms, and watch as your reflection lifts his arm as well. You watch your reflection as he flexes his biceps, the thick muscles bulging into football-sized mountains. Your reflection is grinning. You're grinning. You run your hands down your thick, jutting chest, down your hard, cobblestone abs, to your massive, treetrunk thighs. You bring your hands in, and feel the very full pouch of your posing trunks.
You've grown larger in every way. Your body is taller, a good 6 feet, 4 inches. Your muscles are huge, rivaling the biggest superheavyweight bodybuilders. Hell, you don't just rival them, you exceed them. Your biceps alone have got to be twenty-five inches, maybe more. And your cock. Your huge, beautiful cock. You slip out of the posing trunks, and lay them reverently on the sink. You fear that once they're off, this dream will be over, but no such thing happens. This is PERMANENT. You grab your big, hard, foot-long cock, and stroke it with your over-sized hands. It's thick and hot, and what little pain remained in your body is washed away by the pure pleasure your strokes bring. Your balls look like a pair of eggs, churning power into your glorious cock. With a grunt of satisfaction, you shoot your load all over the mirror, a load bigger and stronger than any you've ever had. You lean forward, and begin to lick the cum off the mirror, turned on by the taste of your cum, and turned on by the sight of your reflection doing the same thing.
Once you finish cleaning the mirror, you reach for the posing trunks. Those wonderful, magical posing trunks. Why did you not want to put them on? You can hardly remember. They fit perfectly, after all!
You slip the trunks on again, enjoying their silky smoothness, and the way that they cradle your balls and half-hard dick. You saunter to your bedroom, and pull out some shorts and a tee shirt. Both are far too tight, but you squeeze into them anyway. The shirt and shorts leave little to the imagination, but technically you're now what society considers "decent". Or close enough, anyway. Shoes are another matter. Your feet have grown much too big for your old shoes. A pair of ratty flip flops will have to do. They're still too small, of course, but they work better than anything else you own. You'll need to buy new clothes, now. But not tonight. After all, you need to be in the park by 8.
You head for the park, ready to thank your secret admirer.