(CW: Mentions of popping, VERY heavy pop-teasing, and some body horror-y size & description at the end)
(I don't normally do or even like popping, but for some reason I was feeling it pretty strongly today so I may as well roll with it. Don't expect it much at all. But DO possibly expect this more detailed puffy look~ I like how bunched up it looks, so you might see it every now and then~)


"HUFF!! Alphys, how do you live out here? I feel like I'm frying just standing in this heat!"
"W-well, I'm just used to it, is all!"
One seemingly regular day in the Underground, a bulky fish monster had just finished moving in some furniture for her reptilian partner. Despite the searing heat of the subterranean cavern, the dark blue, oversized recliner had managed to make it into the lab without neither getting stuck nor catching fire.
"Its a little big but it looks cozy...~"
"No kidding, this thing is HUGE! It could fit like, 8 you's in it!"

Undyne certainly wasn't exaggerating. Sure enough, it was easily big enough to dwarf the two of them. Neither of them really KNEW why it were so large. Simply that it was found in Snowdin. According to what Undyne had heard, there was a thunderous BANG in the lowlands. As Undyne explained what happened next, Alphys' face grew heavier and heavier with rose blush.
"S-so...what you're saying is...someone ate so much they...e-exploded!?"
"Well the guards weren't sure if it was that or a pencil they dropped. I know it sounds pretty crazy, but that's what the outpost told me! Chip bags and yellow monster scraps were flying around like confetti."
"W-were they okay!?"
"Of course they were!! I um, wouldn't be telling you the story if they died. Anyway yeah, they're fine, some healers came and they just gotta spend a few days wrapped up in bandages. Told me she was gonna go twice as big next time and we're ordering a bigger couch to match. Fine by me, gives us more to do waiting for another human to fall!"
Alphys was blushing hard. The mention of "yellow scraps" obviously reminded her of herself. then Undyne sent out a tease that almost made her faint.
"Hey I know you like relaxing but don't explode yet!"
"Bu-Huh I wasn't t-thinking about that!"
"..."
"Promise!"
Undyne just chuckled. She could almost certainly tell Alphys was getting excited over that story.
"Well I've got stuff to do. See you later Alphys!"
"O-Oh! Yeah, see you!
As Undyne left through the left gate, Alphys was left to her devices.
...
...And left to get unreasonably turned on by the idea of being fed until she explodes.
..though given the pencil mention, its entirely possible that monster blew up from that and not from overeating...and if that were the case, what really WAS a monster's limit?
...Did they HAVE a limit?~

She finally figured it out! She could run an experiment! Truly the best kind of science, blatant indulgence disguised as research, she was familiar with it alright~
Some time would pass.
The record button on a tripod camera was pressed, showing the upper level of a dark lab, with only dim lights illuminating the area. To an outsider it'd seem ominous, but this was simply her way of getting into a relaxed mood.
"hello! I am the royal scientist, Dr...Dr. Alphys. I am submitting the following footage as p-part of my research into the...uhm...limits of monster bodies!"
She would go through a script she had written.

"Ahem...ever since the incident in the lowlands, I have been fa-fascinated by just what our bodies are capable of. How strong are they really? How far can they stretch, how much can they hold, how full can we become?"
"That is what I seek to dem-monstrate right n-now as the test subject! You see, I am hooking up this output hose to my body, which will soon f-fill me with...ice cream...~"
She almost got distracted, yet managed to get back on track.
"But yes! Please, do enjoy what you see!"

Alphys said with as much confidence as she could muster as she sat down and shoved the output hose right into her mouth. The ice cream machine got to work, as soon, the pink, sweet substance started getting pumped right into Alphys! Near instantly, her labcoat buttons began to pop one by one. She shivered, both at how fast she was growing, and the sensation of her body being filled with cool, smooth ice cream at such a steady pace!
Before long, her clothes were no more, as she was now cartoonishly round and spherical. Her voice was muffled from the tube in her mouth, but still audible.
"So far its...mmmhf...~ J-just some simple bloating!"
Alphys said, still trying to keep up the veneer of this being research.
She felt her mind grow numb for a moment, whether it be from the relaxation of the sensation, or all the ice cream giving her brain freeze, she felt minutes melt away as she stopped paying attention for a while...
"Mmhfh!? O-oh goshh! Shorry right, I almosht fell ashleep!! W-wait...hold on..."

Alphys tried to regain her composure as she woke up from her trance. Her short body now nearly double its original height and width. As she noticed something strange. Not only did she feel far larger...she felt far FULLER. Not just in her belly, but everywhere!
When she started this "experiment", she figured the only part of her to expand would be her stomach. It wasn't that daunting, especially if she overdid it. At worst, she'd get some bandages around her gut for a while.
...
That was not the case.
Terror and excitement filled Alphys as with each forced gulp, her features puffed out further, her hands had expanded to the size of boxing gloves with useless, taut-feeling sausage fingers. Her feet felt like she was wearing slippers INSIDE of them, and even her spines and tail felt puffier. The pressure was building...~
"Mmmhhhfhh O-okay...shecond shtage shwelling sheems to include expansion of limbs and other tertiary featuresh...~"
She hadn't consciously picked up on it yet, but her voice had begun to deepen. Every minute or so, it dropped another octave. Alphys metaphorical heart began to pump faster as she felt what was coming next.
Pressure building behind her eyes.
"U-uh oh...~"
*CCCCCRRRRRRRRRRRRRRKKKKKKK....*
The first great creak, sounding like a wooden barrel on the verge of bursting open, her gaze darted around, as if trying to re-assure herself that it was simply her seat straining, rather than her flesh.
"Mmmhh...sho...fuwwwlll...Glllllrglll...~"
Her voice involuntarily gurgled as her eyes began to bulge from their sockets, expanding and groaning ominously as they filled with ice cream, their edges becoming bunched up at the base as they slowly pushed out, becoming two bloated, turgid globes that occasionally slapped together, causing the growing mass of lizard to shiver.
At this point, her basic appearance was distorting. Her feet sunk away entirely, her fingers, hands, and arms merged together into swollen, spherical nubs, her head and body started to fuse into a single ball as the bloated flesh outgrew the frame.

"GllLLGLOSHHH...SHooo...TIIIHGHTTTT!!!"
Alphys roared out in her deep, gargling voice, her body was filling to its' max, as the creaking and groaning grew more frequent and loud, she started flailing her nubs around in a panic, yelling and moaning. She was utterly terrified...yet so utterly in love with this state. She was scared of popping...yet loved the thrill, the pressure, the TIGHTNESS~
"MMmmhhhhhhghh! Uh-Uhndoine!! tuwn off thuh ice cweam, p-pweashe!!!!!"
She desperately tried to call out for her crush, fully aware she had already long since left. Even so, its not likely she'd even recognize Alphys' voice at this rate, something which Alphys herself didn't mind all too much~

The rumbling flesh reached a fever pitch as she saw ice cream leaking from her mouth and nostrils. She knew she was approaching her limit. This was it.
"MLRRLGLHH...TLOOH MUWCH ICWE CWEAM!!! GLLRK-"
Thunderous crackling rang out as her hide's seams felt like they were about to pop open.
"AHM GOHNNUH EXSHPWOOOOOOOOODE!!!!!!!"
...
...
And yet, nothing.
Well, not nothing, she was still creaking and groaning INCREDIBLY. But she had not exploded. She was still growing, too. Still expanding, still swelling. Her body felt like it was at its' limits. But it was though something just refused to let her burst! She wasn't complaining. Her mind relaxed to a degree as relief washed over, yet the feelings of peril and pressure still excited her to no end. Even as she waited to see if it may still happen, it never did, even as the pressure became enough to burst open a mountain from the inside~

Minutes melted into hours, as all of Hotland could hear strange, increasingly deep moans and gurgles.
Eventually, Undyne was called to check in...
Upon opening the lab...She saw a near-unrecognizable mass filling nearly the entire lab. A vast expanse of incredibly taut, firm, yet incredibly comfortable, puffy fat All features gone save for those of the face. The only parts of the lab still untouched was the primary monitor, which had various anime episodes playing on it. The masses' gigantic, pickup truck-sized elongated eyeballs were staring straight at it, heavily dilated with incomprehensible joy. Somewhere within, the machine was still pumping more and more...not long after Undyne walked inside, the bloated balloony mass swelled over the doorway out, sealing it just as tight as its own flesh~
...
"Damn it!"
"Why didn't you invite me into this until NOW?~"
Undyne said jokingly as she walked inside, ready to get her fill~ Eventually, the recording would be salvaged from the now obliterated tripod deep beneath Alphys' body, showing her research to the Underground...
...
Needless to say, most of Underground have become ticking couchbombs ever since~