Male with younger brother (gay/muscle growth)

Your name is Masaru, and you have a younger brother named Makoto. You are something of a jock and "big man on campus" at your college, while your younger brother is just a freshman and is somewhat shy and nerdy.

Your brother isn't currently home, but your father called you into his lab, saying he had a new invention to show you. Your father, Tazuna, is a former Sumo champion who still has a huge musclegut build with massive muscles but also a huge gut. When you walk into his lab, you are somewhat surprised to see him wearing nothing except what appears to be a large diaper.

"What do you think, son?" He asks proudly, slapping his belly. "I know it might seem strange a little and you might have questions, but...watch this!" He steps over to a set of massive weights and effortlessly lifts up an absolutely gigantic barbel.

"Wow...but what does that have to do with the diaper?" I ask, curious and somewhat suspicious.

"It's because of the diaper! See, this isn't any normal diaper, it's a bio scanning metabolic maximizer! It perfectly scans the wearer and modifies their body and recycles their waste in a perfected way to achieve their ideal body, to the degree of being able to grant superhuman abilities! It's almost like it can scan your soul, and give you the result that fits you best! Before too long, everyone is going to be wearing one of my Super Training Diapers!"

"So...are you expecting me to wear one?" I say skeptically.

"Haha! Of course! Well, you don't really have a choice..." Tazuna laughs, and then suddenly he seemingly vanishes! You realize he is somehow moving so fast that you can't follow him when you feel him grab you and almost instantly strip off your clothes before you feel something soft and puffy between your legs! He used his new superhuman speed to put a Super Training Diaper onto you and lock it on.

You stumble a bit, feeling slightly dizzy from the rapid change and from now being naked except for a large puffy diaper. Tazuna gives your padded ass a slap. "Now then, we've got some work to do, son!" He says, pulling you over to a training gym section of the lab.