"Master? What're you doing?"
Link looked upward to see the spirit of said sword staring back at him. This strangely beautiful otherworldly being, Fi, had been his companion ever since he started his great journey to save the world, and she was the one he could rely on with almost all of his questions. He showed her the sword, and pointed at the button. Fi floated in a little closer to get a better look.
"Master, I must say that I have never seen this device on the Master Sword before. It is a complete mystery to me as to what would take place were the button to be activated. If you wish, you may try it, but be warned that there is a 50% chance that something may go horribly awry. Please take heed, Master. I will not argue against whatever decision you plan to make."
After having taken careful consideration regarding Fi's information, or rather her lack of information, Link decided to go for it. He gently pressed his finger against the yellow button. For a moment, nothing happened.
Suddenly, before Link even realized what had happened, the length of the blade itself increased. It grew out by several inches, and it felt a little heavier to lift than before. Link stared at his own sword in bewilderment before Fi's voice finally drove him out of his gaze and onto her. He was even more surprised by what he saw.
"Master...I believe that that button you pressed did not only change the sword's form, but my form as well."
Fi's breasts had, like the Master Sword, grown by several inches as well. Her once non-existent cleavage had instantaneously swelled outward into a pair of bouncing, round tits. They would have filled almost any woman of Skyloft with seething envy. They were almost as large as pumpkins, and having had worked part-time at the Lumpy Pumpkin before, Link was well aware of just how large some of them could grow. Her hovering up and down caused the breasts to be caught in a constant jiggle. The young man had already stared at Fi's massive mammaries for nearly a full minute before catching himself.
"Master, I hate to ask this of you, but I believe something must be done to fix this condition. Attempt to press the button again. There is a 59% chance that it will revert this process. I do not think I can remain afloat for too long with breasts this heavy."
Link nodded. He pressed the button on the Master Sword again. This time, the hilt of the sword grew longer in Link's hands. Fi looked over her shoulder to see her butt had expanded now in response to this second alteration. Rips and holes were littered all over the surface of her cross-stitch leggings as her legs and hips followed suit. Fi, contradicting her usual quiet and composed nature, let out a tiny moan. Her flat butt was a pair of perfectly-round spheres, large and heavy, bouncing nearly as much as her breasts as she struggled to keep herself in mid-air. Her dress was forcefully pushed up all the way due to the size of her ass, allowing the meaty cheeks to be seen in full-view by her master. However, Fi tried her best to retain some of her dignity by completely ignoring this particular issue.
"M-Master...I implore you once more to press the button again. I am confident that another press may be able to revert these changes. There is nothing else that can be altered to the Master Sword now. Please hurry, for I fear I may fall soon. There is a 60% chance that everything will return to normal."
Link, who was beginning to doubt some of the sword spirit's predictions, still obliged himself to follow her request. He pressed the button a third time, and now the wing-like adornments on the sword's sides sprouted outward and took on a more realistic shape and appearance. Link quickly whipped his head up towards Fi to witness her next transformation. Fi's entire body began to tremble violently all over, which also prompted her breasts and bum to jiggle even more. Link's eyes bulged to the size of dinner plates as his partner's stomach started to inflate, like a balloon.
"Ah!"
Fi's dress burst apart in an explosion of fabric that Link didn't even know existed. Fi's slender, blue face puffed up with fat, giving her chipmunk cheeks and a nice double-chin that hung low over her neck. Link actually found it to accentuate her cuteness somewhat, but this was not the time to be admiring such things. Fi's porky breasts filled out even more to the point that gravity could no longer contain them. Fi winced at the sudden surge of pain that spread throughout her spine. Her back became enveloped in rolls, popping up out of her skin like many little buds. A pair of scrumptious love-handles, five-inches thick, wrapped around her whole waist, enveloping it underneath her own fat.
Fi's flat, feminine abs twisted and contracted and it was flowing out in all directions. It split into several plump rolls stacked one on top of the other, the bottom-most one less than an inch away from going over the waistline. Many tiny dimples speckled this particular roll of her belly. Her legs and butt were now bigger than ever, larger than life. Most of the weight she had gained went to these specific areas of her body. Her butt was big enough that it could have eclipsed the thin version of herself, covering her in shadow. Her legs, when pressed together (which seemed to be a permanent thing now) were three and a half feet across. The way all of the fat on her legs shifted in unison so perfectly, it was like they were melding into one.
Fi finally gave into the laws of physics, and with a defeated sigh, her huge, quivering form crashed down to the floor.
Link's jaw would have dropped to the floor along with her, if it had been capable of doing so.
"Master...please...I think it would be best...ooh...if we found some other way to reverse this...without pressing the button...a 100% chance..."
Link nodded in agreement. However, when he attempted to move away, he realized that his sword was inexplicably weighty all of the sudden. Far too heavy to even lift anymore, in fact. Apparently, Fi's weight reflected the sword's weight, which, in some sort of alternative understanding of the world, could have made sense.
Eventually it was discovered that the sword had been 'secretly modified' by Gondo, the Scrap Shop owner, under the request of Scrapper, the snarky little robot that had had a crush on Fi, and a hatred for Link, for some time now. Apparently the ancient machine had been able to pick up enough rupees from the world below to afford the work to be done, and Link was none the wiser to it.
Link would have gladly broken Scrapper into pieces himself, if only his sword and the spirit concealed inside of it hadn't both weighed over 500 pounds.