Arceus took in the gazes of the gym goers, who had stopped to stare at the muscular bulking that he just underwent. It was clear they were not just shocked by his musculature, but also enamored by it. He couldn't help but grin, and then begin to flex his increased bulk. this seemed to please nearly everyone there, minus the Machoke who was so rude earlier. He then got an idea for how to impress these muscle fans even further. If they enjoy seeing him become bigger, then why not become bigger while flexing? After all, it only made sense. He could see they wanted him to show off more, so why should he deny their wish?
There seemed to be a brief pause as Arceus took in a breath. His chest swelled with muscle, and he was sure he heard one or two gymgoers gasping at how his outfit tightened across his chest. It was understandable, of course - he'd grown from four-foot-something to easily five and a half feet tall in mere minutes, and fully doubled his weight and musclemass in the same time - but he couldn't help but lick his lips at their reaction, a giddy tingle racing up and down his spine. If they thought this was shocking, then they had no idea just how far he was planning to go - and how much farther than even that he could go, if he wasn't occupied with putting on a show.
"Phew! That was a good workout!" Arceus loudly announced. His voice seemed to echo in the silence that had fallen over the gym, and now the Machoke from before finally lifted his head. He seemed to realise that something was wrong over the course of a few moments - first noticing just how silent it suddenly was, then realising that everyone was staring at the short twig from before and not him. "Done with those one kilo weights, yet, shrimp-" he said, turning towards Arceus - but fell silent, and then after a moment his eyes widened and his mouth fell open in shock, the heavy weights falling from his meaty hands with a deafening thud.
Arceus let out a little laugh at the reaction. Something halfway between a giggle and a chuckle. His voice was still a little bit high-pitched, though at the same time much deeper than before. He winked, tongue poking out slightly as one hand cupped his muzzle overly-innocently, the other tracing lines over his expanded musculature. "I am!" he said excitedly, as if nothing was out of the ordinary. Well, nothing out of the ordinary for mere mortals. For a god like him, this really was as mundane as could be. "I don't think I'll be getting much more use out of those little weights, anymore... Maybe we should see just how pumped I am after that workout?"
The Machoke just wordlessly stared as the Arceus took in a deep breath. His already tight clothes making stretching and straining sounds as he flexes his muscles, bulking up even further from the action. He does this for a bit a minute, choosing a different pose for flexing each time. And he slowly bulked up higher and higher, getting wider and wider. His shirt became so stretched and strained, that it was becoming veritably see through. His pecs pulsed larger and larger, growing more defined as well, as his flexing routine just made him grow bigger and bigger and bigger! At least, that's all they could see for now. Really, he could make himself so much bigger, so much faster, but it would hardly be a good show if he did it like that.
"Ah, that feels good." He says, flexing his right arm's biceps, while everyone else just stops and stares, jaws agape. Arceus had more than easily become one of the tallest people in the gym, standing at seven feet and three inches tall. Not to mention how wide he's become, being almost as wide as he was tall. "So, what did you call me earlier? I believe it was 'shrimp' or something?" He says teasingly.
Choice 1 - "Yeah! Yeah, I did!" the Machoke shouts, not to be outdone, angered at this challenger - and what's more, his own flexing makes him grow, just like Arceus!
Choice 2 - "U-uh - I mean, I, uh..." the Machoke mumbles, completely cowed by Arceus rapid, massive increase in size and power. He's part of Arceus growing audience, now...
Choice 3 - "..." The Machoke is silent- cheeks flushed, a flutter in his chest. Try as he might to deny it, he's completely smitten by Arceus' growth, his new size and strength - and wants him to grow even greater...