Per One Piece Wiki site, the following was an SBS question in Volume 78, Chapter 777:
D: We realized that Robin has huge breasts.
We checked from the side, and Nami also has huge breasts.
Why do they both have such huge breasts?
And could it be Oda-san's preference?
That was a question from us two 10-year-olds.
P.N. Sakucchan & Yuzu-chan
O: I keep saying this, but I'm just bringing the young boys' dreams to life! All of humanity on earth! I command your breasts to grow~!
/
Oda-san may not have this kind of power in our world, but what about the One Piece world? He also mentioned in another SBS that he only knows how to draw one body type for girls so let's not be too hard on him; drawing people is hard.
Please disregard the OP timeline and the fourth wall with this story. Thank you for your time.
Sanji awoke, early in the morning. Time to get breakfast ready. He stretched but suddenly stopped. Something was off. It felt like there was an immense weight on top of his lungs. He sat up. It didn't feel like heartburn. What on earth… Sanji reached up towards his chest.
Strange, he thought, these feel kind of like Nami-san's. But, he reached lower, these feel like mine. He reached back up towards his chest. And then back down between his legs. He was motionless for about 3 seconds before he immediately bolted to the bathroom to find the mirror.
Meanwhile, aboard the Polar Tang:
"Caaaappttttaaaaaaiinnnnn!" Bepo screamed, "What happened?!"
"It's called gynecomastia," Trafalgar Law calmly stated, "Although it's strange; the onset shouldn't be just overnight-"
"Caaapppptttaiiiiinnnnn!" the rest of his crewmates' screams echoed throughout the submarine.
"-nor should it affect an entire crew of people as if it were contagious."
The den den mushi rang. Law picked it up. "This is Law."
"Traffy!" Luffy sobbed through the snail, "Why does everyone have boobs?!"
"Doctor! We need to see a doctor!" Chopper shouted in the background.
"You guys, too, Mugiwara-ya? This is sounding like an epidemic. Maybe even a pandemic…"
Meanwhile, aboard the Moby Dick:
"Ace, please put a shirt on, yoi," Marco covered his eyes.
Ace frowned. "I told you, I don't want to wear any shirt 'cause it'll cover up the awesome Whitebeard tattoo on my back."
"Yes, but it's different when you have a pair of melons on your front, yoi."
"Have you seen Pops yet? His are beyond melons," Thatch walked up to them. "Like, I don't even know if we should be calling him Pops anymore."
"Well, we can't call him Mama; Big Mom's already got that name. Wait, do you think hers and all the other girls…?"
"Yup. When I went to go find the nurses earlier this morning, all of their breasts have grown even larger, yoi."
Meanwhile, somewhere not Baltigo since that place got wrecked:
"It wasn't me, I swear!" Ivankov protested.
"Are you sure? Because everyone knows about your ability to alter hormones," Dragon glared at Ivankov.
"I would change your genders, though! I wouldn't just tack on a pair of breasts to everyone! You have to believe me! Sabo-boy! You believe me, don't you?"
Sabo looked up from his conversation with Koala. "Hm? Yeah, sure. You know, Koala and I think we might know who's behind this."
Meanwhile, aboard the Going Luffy-senpai:
Bartolomeo dialed the number for their beloved all-knowing Grammy. "Hello? Grammy? It's Bartolomeo. We all got boobs all of a sudden. What do we do? Mm hm. Mm hm. Ok, thanks, Grammy!"
Bartolomeo turned and faced his crew. "Listen up! Grammy says that if we can't fit two fingers through the back strap, then our bras are on too tight."
And so, everyone on the ship spent the rest of the morning trying to reach behind their backs.
Meanwhile, at Saboady:
"What the crap is going on here?!" Roger shouted as he burst into Shakky's Rip-off Bar. His hands were gripping his head as if his skull would explode if he didn't keep it clamped.
Rouge gave a sympathetic smile when she saw that Rayleigh and Shakuyaku were both dealing with the same problem as them.
"I can't even cross my arms! Look, I'm trying to cross them right now but these-these-things keep getting in the way!" Roger struggled with his arms and cleavage for common ground but to no avail. "Rouge, how do you usually cross your arms?"
"Your arms sit under your chest, dear," Rouge demonstrated by crossing her arms gracefully over her stomach.
"But that's not high enough! I need them higher so I look tougher!"
Rayleigh and Shakuyaku laughed. "Never a dull moment with Roger," Rayleigh said. "Say, how about riling up some more excitement with us?"
"Eh? What are you guys planning?"
Shakuyaku grinned. "We have an idea whom might have done this to us."
Meanwhile, wherever the hell Shanks is:
"Hey, Mihawk."
Dracule Mihawk didn't respond; he had a feeling that Shanks wasn't going to say anything worthwhile.
"I bet mine are bigger than yours."
Mihawk sighed. He stood up and walked away.
"Wait, wait, Mihawk, I'm sorry! Don't leave me~" Shanks feigned sadness as he reached out towards Mihawk.
"I only know one man who could be capable of this," Mihawk said without breaking his stride. "I'm going to pay a visit to his office."
Meanwhile, back on the Thousand Sunny:
Sanji twirled around Nami and Robin. "Nami-swan and Robin-chwan look magnificent~!"
Nami made a face. "My bras don't fit. Again."
"Hmm," Robin placed a finger on her chin, "I think I might know who's responsible for this."
At that time, the den den mushi rang. Luffy went over to pick it up.
"Hi, this is Luffy, the future pirate king but I might end up being the queen because I can't get these boobs to go away."
"Luffy, it's Sabo," the voice on the other end said as the snail took on Sabo's facial expressions. "I have an idea as to who's the culprit. Want to help me round up everyone we know to have a talk with him?"
Meanwhile, at Marine HQ:
"Sound the alarms! Gather up all your weapons!" a high ranking officer shouted at the other officers who were all frantically running about in every direction, assembling together.
Coby and Helmeppo gawked at the amount of soldiers and weapons gathering. "Wow, is this large of a force necessary?"
"Yes, it freaking is!" shouted Garp, who was standing at the front of the amassed army. "He's got to learn his lesson! All units! Advance!"
All the marines began running towards their destination, armed with every weapon available to them. Coby and Helmeppo were keeping pace with Garp at the front, albeit uncomfortably.
"Do these things ever stop jiggling?" Helmeppo asked out loud. Redfaced, Coby tried to hold down his own chest while they continued their march.
Meanwhile, at Oda-san's office:
"Er...Oda-san? There are some…visitors? For you?"
Oda looked up from his most recent chapter he was working on. "Eh? Is it really appropriate to be having visitors right now?"
At that moment, Luffy pushed his way through the door. "You! Change us back! Now!"
"What the-?!"
Soon, the entire office was flooded with One Piece characters angrily demanding their original bodies back. Oda could only stare, amazed that his creations have come to life and are physically standing in the same dimension as him and…
"Wow, look at all these knockers in one room. I feel so rejuvenated, I might even be able to work twice as fast-"
Nami grabbed him by his collar. "FIX. THIS. OR. DIE."
"Yes, Nami-san. Very sorry, Nami-san. Won't happen again, Nami-san."
And so, Oda returned everybody's bodies back to normal and apologized profusely to all the characters. He was sternly warned to be more careful with his words and once satisfied, everyone peacefully left except Luffy, who raided the fridge.