Caleb pushed his muscular ass back against Luke, gently squeezing his glutes around Luke's giant cock. Luke grunted approvingly and started to slide his huge dick back and forth, pumping it into Caleb. The smaller man moaned with pleasure, a strange sensation washing through him. With every thrust of Luke's big dick, Caleb's muscles tensed, the hard bulges springing into relief under his thin skin. Both men noted the effect: it seemed that Caleb was growing. Luke looked down at Caleb's broad back, his defined traps. They were clearly bigger than before.
Luke paused, keeping his dick still, the big rod driven deep into Caleb. When the fucking ceased, so did Caleb's growth. Luke waited a good minute, keeping himself buried inside Caleb, his big, rough hands roaming the smaller man's finely-muscled body. Caleb moaned softly as Luke felt him up, but he didn't grow.
Then Luke slowly started sliding his dick back and forth once more, plowing Caleb with deliberate strokes. He observed carefully, and saw that Caleb's growth was starting up again, the smaller muscleman gradually becoming less small, his body packing on more and more muscle as Luke thrust his dick in. Luke was torn. He liked being the bigger, stronger, dominant one. But seeing Caleb grow was hot as hell, and the bigger Caleb became, the hornier it made Luke. Knowing that he was growing Caleb by fucking him was a massive turnon for the larger man, and he ran his hands over Caleb's body as he pounded his dick into it, feeling the hard muscle grow under his palms.
"You feel that, Caleb?" Luke growled, "You feel that body of yours getting bigger as I fuck you?"
"Fuck yeah... feels so good..." Caleb breathed in reply, running his hands over his own body, feeling the muscles growing under his skin.
"You want me to keep going? You want me to make you huge?" Luke purred, leaning in to breathe in Caleb's ear as he fucked him.
"Fuck yeah!" Caleb replied, grunting enthusiastically, "Never thought I'd love getting fucked so much! And this muscle... I didn't think I wanted to be this big... but now... fuck! I love it!"
That was all it took. Luke started pounding Caleb's sweet, increasingly-muscular ass with hard, determined strokes. The harder he slammed his dick in, the more Caleb's muscles responded. Luke grunted approvingly, watching Caleb expand, the smaller man growing until he had matched Luke's size, and then exceeded it. Luke kept going though, his dick hard as could be, aching for release. Caleb grunted in pleasure, pushing his huge body back against Luke, loving the twin sensations of having his ass ravaged and having his muscles grown. Luke was going nuts, ravaging Caleb's ass, roughly thrusting his oversized dick into the hole between Caleb's fantastically-muscled ass. Luke wanted to go forever, but his aching balls demanded release as well. He let out a loud cry as he finally came, his dick spasming madly, spewing a hearty load of cum into Caleb. Caleb's muscles exploded in size, swelling dramatically as he drank in the strange energy released by Luke's orgasm. Luke collapsed against Caleb's massive back, holding onto it for support as his dick pumped out the last bits of his seed. After a minute, he slowly pulled out and stepped away.
The sight that met his eyes was beyond impressive. Caleb loomed, six feet and seven inches of massive, brutal muscle. He had to be close to 400 pounds, Luke estimated, every bit of it huge, hard, striated muscle. Caleb stepped away and turned, giving Luke a good look at the giant, rigid dick that jutted from Caleb's groin - it was at least fourteen inches long, fantastically thick and veiny, with a pair of huge balls hanging beneath it. Luke's mouth went dry at the sight. Caleb grinned and raised his massive arms, slowly flexing them, his biceps peaking spectacularly, the huge balls of muscle straining for release under the thin skin, amply adorned with veins.