5'6" Nerdy pudgy dude, just graduated college
Lives with in a house with a jock still in college
Roomate goes out for whole day, dude is home alone
dude goes about day
Pudge starts to melt away, keeps adjusting belt and shirt
Posture corrects and tightens
Has extra energy, Decides to do some yardwork, its easier than expected but has to keep adjusting belt, shirt keeps untucking
Done with some of it, decides to go shower
Pants and underwear literally fall off of him as he pulls off shirt, doesn't realize
Examines self in mirror, 5'7" and lean, light dusting of light body hair
Seems puzzled like something is off but shrugs, goes to shower
Showers, rubs with cleaner all over, mental rundown of body
Feels tightness in balls, rubs them [descibed small], then decides to stroke off.
Faps to completion in shower, washes off again
gropes larger balls, "Naughty boys making me do more work"
Towels off, examines in mirror, 5'9", slightly toned, larger frame, body hair is thicker
Puts on pants, tightest setting they barely fit. Goes commando, doesn't realize. 
Lunch time, he goes to make Meal A, pauses confused, then goes for Meal B instead.
Puts on music, it's classical, changes it to something more upbeat
Goes out to do more yardwork
Hey this seems easier now
Has to adjust his pants and shirt, expanding hte belt a few times and loosening the shirt.
Yardwork is done, pants and shirt feel too tight like they shrunk
Shirt tears at the armpits and down the sides putting stuff away
Pants split in the seat down the middle when he bends down
Sighs, goes inside, and takes off clothing (causing more rippage)
Looks down at now very hairy and built form, 6'0
Tries to find his gym shorts that fit, can't, goes into roommates room to find his
Oh wow um leather harness stuff, didn't know he was into that, not my thing at all
Snags some workout clothes, loose on him
Dinnertime, decides to make steaks, keeps changing his mind to how he wants them from well done, to rare
Decides to watch show while eating, his normal one seems eh, but his roommate was talking about this other one
Cartoon action staring a big built white tiger barbarian and a twink cheetah mage. Totally homoerotic
Dude is gay, but prefered twinks, but while he admires tiger's form he's more into the cheetah
Looks at tiger and thinks "I wonder how I'd look dressed up like that"
Eats his steaks, his mouth stretching into a muzzle as he bites in and chews it, his ears stretching
Blinks, eyes shift to cat iris, realizes that the harness he saw in roommates room looks a lot like the tiger's
Gets up and rubs his back and bottom through the tight gym shorts, adjusts to let his tail out
Goes into roommates room, tries to strip off shirt, accidentally tears it off because it was tight. Snarls and just rips off the shorts, too. Admires 6'5" hulking form, his body hair turned into short fur that does nothing to hide his muscles, light grey stripes slowly fading in
Goes to put on harness. Becomes more and more skilled with it as he tries to put it on, realizes it's on the smallest setting for his roommate, lets it out a few times
Puts it on, has to adjust it again to the next largest, then as he feels it fits it suddenly tight again, so continues adjusting until it's max size
Puts on the full outfit, grins and checks himself out in the mirrior, a 6'6" beastial white tiger barbarian grinning back at him
Ear twitch as his roommate comes home and calls out
Strides out with full gear on, roommate's jaw drops "Wha"
Dude notices roommate has a boner, decides to get in character a bit and force him over
Oh yeah Dude has a big thick boner already and the loincloth aint hiding it
Roommate is pulled down and hesistantly licks the tip, Dude growls and roughly facefucks him, "I've been wanting this a long time,"
Dude chuffs and growls loudly, roommate shrinks down in size going from jock to twink
Dude growls, rips the roommate's baggy clothing off, "Yes, a long time [Character name]."
Roommate goes "Wait what" but is interrupted by 10 inches of thick tigercock going into his ass
Roommate's changes accellerating, cheetah fur growing out, muzzle popping into place, growing sense of confusion along with building pleasure
"Oh gods [Character name], make me yours!"
Dude hilts roommate and gives a mating bite, cheetha's belly bulges with tiger seed
Recover, they're both fully the characters, don't know where to go from there.