Another idea that's been swimming around in my head. This is mostly because there isn't enough fat King Kazma in the world and I aim to rectify that with this weight gain story. Hope you like it!

“Attention, avatars of OZ. We are holding a special competition today for only the finest and most seasoned challengers in the world. Anyone who thinks they’re skilled enough to endure our perilous challenge, please enter the special cube placed in the center of OZ Square, the public congregation ground for OZ denizens. Do you think you can take our special mystery competition?”

King Kazma, the goggled, fighting rabbit of OZ, was never one to turn down a challenge, even if he didn’t know what it was he was participating in. He couldn’t help himself when he heard the announcement of a special mystery challenge and had to see if he was up to it himself. He flipped the collar of his red vest up, knocked the front of his sneakers on the ground, and entered the large cube mentioned in the announcement.
To his surprise, he found himself in a closed-off white room, with no one around him.
“What kind of challenge is this?” he asked to no one in particular. “There’s no one here.”
The King glanced around suspiciously, waiting for someone or something to pop out and challenge him to fight. His ears were pricked and rotating like antennas, scanning for any sound at all.
“WELCOME, KING KAZMA,” a shrill and excited female voice sounded from seemingly nowhere.
Kazma looked up in surprise and tried to look for the source of the voice, placing himself in a fighting stance.
“Who’s there? Show yourself!”
“No need to take this so seriously, your majesty! I’m only here to introduce you to the challenge!”
“Oh,” he said disappointedly as he slowly let his guard down.
“Much better. Anyway, *ahem*, welcome to the mystery challenge! Here are the ground rules: You can’t participate without first giving us your consent and there’s no leaving once the challenge has started unless you give up! Other than that, just try to win!”
“What, you’re not going to tell me what the challenge is?”
“Maybe I will, if you accept it first!”
Kazma wasn’t sure what this strange girl was playing at. He felt a bit unsure about accepting a challenge first and finding out what it was later. “But what kind of king would he be if he turned down a challenge?” was his thought, however.
“Okay, fine. I accept the challenge.”
“I knew you would,” the girl said excitedly.
“Let the challenge begin!”
A section of the floor opened up and from it popped out a long table filled with delicious-looking but fattening food.
“What the? Is this an eating contest or something?”
“Just eat as much as you can until I say stop.”
“No, thanks. I’m a fighter, you know. I gotta keep my weight down.”
“Oh, I’m sure a fit and talented rabbit like you can work those pounds off in no time. Besides, you already accepted the challenge!”
She did have a point. He didn’t want to just quit when he hadn’t even started the challenge, and he would be able to work off any of the weight he was sure to gain.
The King also decided that he probably wouldn’t eat that much anyway.
“I guess I should get started, huh?”
“Yay!” the voice cheered. “Good luck, King Kazma!”
A chair appeared at the foot of the table as Kazma sat down in it. He sighed at the amount of food on the table, knowing he couldn’t possibly finish all of this. Nevertheless, he got started with a nearby cheeseburger.
After he finished one, he went straight for a slice of pizza. Then some sushi. Then spaghetti. Then a slice of chocolate cake. He settled for taking a bit of anything and everything within arms’ reach, just to say he ate a good portion of the feast.
He had to admit that the food was pretty tasty. He wasn’t used to eating so many fatty foods, but he was glad to at least try it here. Why, it was so good, Kazma almost found it difficult to stop eating. His self-control took over soon enough, however.
After a few minutes of constant eating, King Kazma finally halted his little binge with a belch and a pat of his slightly rounded belly. He took notice of his pants feeling slightly tighter around his waist and looked down to his widened waistline with disappointment and frustration.
“Maybe this wasn’t such a good idea,” he thought. “This little gut will be difficult to work off. Nothing I can’t handle, but still.”
“You’ve stopped,” the girl’s voice said. “Is something wrong?”
“I think I’ve had just about enough,” Kazma said, bringing himself up to his feet.
“What? But you’ve barely eaten a thing!”
“Are you joking? Just look at me! I’ve practically stuffed myself silly! My gut is almost hanging over my pants,” he exclaimed as he rubbed his full stomach.
“That’s a shame,” the voice pouted. “We’ll have to fix that, won’t we?”
“You can say that again. Anyway, I tried out your little challenge, but I think I’m calling it quits. I gotta exercise a bit.”
“Oh no, you don’t,” the girl teased.
“Hm?”
The rabbit’s ears perked at the girl’s words and tone. Though that became the least of his concerns when his chair suddenly slid under him, forcing him back down into the seat. Rings on the legs and armrests kept him from trying to escape as the chair slid back to the foot of the table.
“Hey, what’s going on?” Kazma asked worriedly. “You said I could leave once I gave up!”
“That’s true, but I’m almost certain that you can do better than this,” she said with a sly, expectant tone.
The King struggled to escape his chair to no avail. He almost failed to notice the food beginning to float in mid-air. When he finally saw it, he stared in shock.
“Why is this food floating?”
“Well, with your hands all tied up, how else are you going to get fed short of me coming down there before I’m ready?”
“What!?” King Kazma exclaimed. “What kind of eating contest is this?”
“I never said this was an eating contest, did I? This is more like a test to see how big a bunny like you can get!”
Suddenly, a dozen donuts flew right towards his mouth. The King tried to shift his mouth away, but two metal bars appeared on both sides of his head, keeping it perfectly still. The King tried keep his mouth shut, but the seat began vibrating, sending a tickle throughout his body and forcing a laugh out of him.
Before he knew it, each donut found their way inside of his mouth, stuffing his cheeks. The two metal bars restraining his head retracted, not that it did him much good, considering his mouth was already stuffed, too stuffed to try and spit anything out, so the worried rabbit had no choice but to start chewing and eating his way through each donut. Immediately after he was done with the donuts, however, even more food flew into his mouth, effectively creating an endless stream of food being forced into his body, which began experiencing the consequences.
With each swallow, his belly started growing and filling out. If it didn’t hang over his pants before, it did so in earnest now, as his lap was slowly being overrun by his growing belly. His growing waistline began developing lovehandles that were flabbing over the sides of his pants.
Speaking of his pants, they began feeling tighter as he chewed and swallowed in an endless fashion. He looked down to see his thighs blowing out, and his butt practically inflating with fat. His fluffy tail twitched as his pants practically began receding under his growing rump. He heard a couple of rips and tears; probably his pants being ripped apart at the seams by his thighs and legs growing ever-so-flabbier. The chair even receded into the ground, finding it pointless to restrain a fattening rabbit that soon wouldn’t be able to move of his own accord.
His arms started growing their own flab. As Kazma struggled, his fattening arms began jiggling and jostling with fat. His red vest, which used to cover his whole torso, now tightened around his growing moobs, while barely making an effort to cover his giant belly.
King Kazma began moaning at his pants becoming tighter and tighter around his rump and thighs, until eventually, they ripped clean off, leaving him almost naked were it not for his vest and white undies, which were practically a thong at this point, his growing butt even spilling out of that.
As King Kazma grew fatter and fatter, the girl couldn’t help but giggle.
“This is working out better than I thought! Look at how fat you’ve gotten, my big bunny!”
The food entering Kazma’s mouth stopped just long enough for him to get a few words in.
“You…was this your plan all along? To lure me into this room and fatten me up?”
As he finished speaking, the food continued to stuff his belly and blow out his features.
“Guilty as charged! Now, if you’ll excuse me, I have to look on at my work in person!”
A door opened, revealing a lovely, beautiful rabbit avatar that looked similar to King Kazma dressed in red spandex.
“Who are you?” the King muffled through his eating.
“You can call me Queen, if you like. And you’re probably wondering why I’ve decided to stuff you full of food and fat.”
King Kazma glared at her as his body continued to swell.
“Well, I’ve been watching you for quite some time. I have to say, you’re quite handsome and oh, so charming. But all I wanted was to have you to myself. I figured what better way to do that than to render you immobile by making you as fat as possible. The best part is we both get a brand new snuggle partner! You’d enjoy that, I think.”
“Enjoy it!?” he muffled. “How could I possibly enjoy being fattened up so much that I can’t move!?”
“Why don’t you see for yourself?”
Queen jumped onto Kazma’s belly, massaging and kneading it like dough. As he continued being fed, King Kazma suddenly felt very relaxed. The feeling of his belly being gently pushed and snuggled with made him feel especially warm. She then began shaking his lovehandles and jiggling his body. He both moaned and laughed at the ticklish feeling. Queen then went behind him and started rubbing and massaging his fat rump and enjoying every second of his fat being massaged.
“Now how did that feel?” Queen asked.
With a sigh, King Kazma happily began eating the rest of the food being forced into his mouth. He suddenly became acutely aware of his body being fattened. His belly continued blowing outward, his large thighs were almost being absorbed by his even larger rump, and his vest had finally given out to his moobs as it tore in half and fell to the ground. The only pieces of clothing that remained on him were his goggles, sneakers, and underwear, which barely contained his giant butt.
With one last donut, all of the food was done, and King Kazma was stuffed to the point of immobility. His previous toned figure was a thing of the past. It was eaten up by his massive belly, huge butt, thick thighs, flabby arms, and giant moobs. For fun, he tried to get up and walk, only to find himself squirming under his fat.
And suddenly, King Kazma smiled.
“I feel…” he began, “I feel so good. I feel warm and fat and lazy and I love every bit of it.”
Queen climbed up to King Kazma’s fat face and gave him a kiss on the lips. She then settled on his soft belly.
“I’m glad you see things my way, because I’m gonna keep you nice and fat for a loooooong time.”
“I like the sound of that, Queen.”