He slowly positioned himself behind the 400lb stud. Gripping onto Caleb's massive back muscles, Luke began deliberate, rhythmical thrusts with his enlarged penis. He could feel the growth working already. As he penetrated the stud's huge muscle ass, every inch of Caleb's body began to swell bigger, his expansive back muscles growing to titanic proportions with deep ridges forming. The comparatively tiny waist began to expand under Luke's palms, bowing out as Caleb's waist began to swell into a firm muscle gut. Luke grunted approvingly as the 6' 7" powerbottom grew larger.
"mmmhhhh fuck you like that Caleb" Luke grunted between thrusts.
Caleb could do nothing but moan in a deep, basey rumble as his body expanded, Luke's disproportioned dick feeling smaller inside his growing ass.
The bed began to creak with the strain of the titanic behemoth. The metal supports that held the mattress up began to buckle, unable to hold the colossal Caleb as every muscle on his body expanded bigger. The hulking brute placed his giant hands on the floor to the side of the bed, his arms quickly growing with the rest of his body as he overtook the kingsize double bed in muscle mass. Luke had to readjust his stance as Caleb's bulbous muscle ass clenched itself over his dick, the perfectly smooth bubble butt pressing against his waist as it took even more of Luke's member. Luke, now the smaller man by a long shot, had to spread his arms wide just to hold on to the sides of Caleb's fantastically bulging muscle gut.
"Fuck dude .." Caleb rumbled. His voice much lower than before. A thick coating of hair grew around the hulk's chin, giving him a full grown beard in a matter of seconds. His face began to sculpt itself into that of a 25 year old handsome muscle beast with short black spiked hair, shaved neatly at the sides that complemented his squared off face perfectly.
By this point Luke had to stand on tip toes just to reach the level of Caleb's muscle ass. It threatened to lift him into the air, attached to the muscle giant by nothing but his penis, Luke tightened his grip at the base of the raided stud's freakishly huge apollo's belt, which gave the effect of muscular love handles forming the base of the giant gut. Caleb's pectorals too were swelling bigger, becoming so big that he rested on the bed with nothing but his ball gut and mammoth pecs. Biceps the size of volleyballs and shoulders the size of wrecking balls held him over the bed, which seemed to be giving up the fight against the titanic god growing in the bedroom.
"OOhhh shit dude, I .. I'm gonna cum!" Luke cried as his dick unleashed a torrent of semen inside Caleb's gaping hole. A loud moan that shook the windows could be heard through Luke's cry as he pumped load after load into the giant man's ass. The beast drank it up through his prostate, packing on the last of the growth as the bed finally gave way. Caleb did not have far to fall, as his enormous muscles had swelled to the point that they were practically touching the ground with their size. Luke dangled in the air, his dick rapidly shrinking back to its regular (but still impressive) size of a foot long. As it shrank, it popped out of Caleb's ass and he tumbled to the floor.
The floor was surprisingly sticky. Luke realised that Caleb must have came at the same time as he did, flooding the hardwood floor with his cum. Low, heavy breathing emanated from Caleb's direction, and Luke stood up to gauge the extent of the muscle stud's growth.
The 400b stud that was previously perched ass up on the bed was no more. In its place was an eight foot six inch, nine hundred pound muscle behemoth with balls the size of large burlap sacks and a cock beyond the size of a normal man. Luke walked around, marvelling at the enormous redwood thighs of the muscle god that lay before him. He pressed lightly into them, feeling their firmness as they bulged with power. He gingerly navigated the gigantic forearms, the biceps of which looked as though someone had stuffed huge basketballs under the skin. He reached the front of the hulking behemoth, gazing upon Caleb's studly bearded face amongst the swollen traps and engorged shoulders.