Here is another roleplay with growth as the center theme, this time with  flitchard
Enjoy!

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"Move, move, move!" The human poked the dog sized lizard with a spear, urging him to walk on the corridor. "You're next! Into the P.R.E. chamber!"

The lizard, Zora by his name, hissed angrily, and even stuck out his tongue. "Alright, I'm moving! You don't have to be rude..." He barely reached the ankle of the man, yet he was not afraid of the bigger creature.

"Less talk, more walk!" The man growled.

Zora sighed, but he was already on his feet. Here comes another fight! He could already hear the chanting of the crowd from the grandstand of the stadium.

Since the dawn of time humans kept magical and speech-capable beasts as pets, servants, or in Zora's case, cage fighters. The lizard was born in the corridors of the quarters beneath the arena, and taught to fight other animals of the same misfortune in the name of entertainment. People came to the arena to see the fight, place bet, win and lose money - and if the animals put on a great show, they were fed and kept well. If not, the punishment was severe.

Zora knew nothing else of the world other than the arena. He heard that humans terraformed the outside world as well, by building skyscrapers, inventing electricity, connecting cities with roads, and so on. But here, it was not much else: eating, sleeping, training, and of course, fighting.

The only invention that reached the arena was the very same invention that boosted the interest of cage fighting animals among the humans. It was the soul and heart of the arena. The central piece of their entertainment. They called it the Physiomorphic Recalibration Emitter, or P.R.E. for short...

But in practice, it was a growth ray.

Illegal in all states and all across the globe, yet still used in multiple underground fighting arenas, the owners of the place always explained that the growth ray was a blessing to the animals: it empowered them, and literally blew them to the size of monsters of the ages. They should be thankful, earning such a power! But for Zora, it was more like a curse. Being a 30 feet tall lizard giant was indeed intoxicating, but it also reminded him how vulnerable he was to the humans. Even with all that might and strength he was still a prisoner!

And the man behind him with the spear reminded him every minute. Zora entered the chamber, and waited for the large coil-like machine to whir up, aim, and shot him with a crimson colored laser beam. Then a hydraulic pressure plate knocked him out of the chamber before the transformation began.

Zora stood up with shaking legs, and he already felt the familiar warmness spreading through his limbs. The crowd cheered and went crazy as they witnessed the once dog sized lizard bulging and swelling, doubling in size in mere seconds, then doubling again, and growing further and further. Zora clenched his teeth, submitting himself to the transformation. He had no choice. The dirt was pushed aside under his swelling feet, his tail lengthened and thickened, his head ascended up, closer to the thick reinforced bars that surrounded the dome-shaped arena.

When he reached 30 feet, and his growth stopped, he looked at the crowd. There were hundreds, if not thousands of people, each one smaller than a single one of his paws - yet behind the bars they were completely safe to watch how two colossal beasts soon punch and kick each other. The announcer hyped them even more, but Zora was not paying attention.

He took a deep breath, watching the other side of the arena. Better be focused; if the owners are not satisfied with their behavior, they're gonna use the ray in reverse and with more intensity, shrinking them down to the size a fingernail for a week. He doesn't have to win, they just have to put on a good show - on an epic scale.