Oda answered this question himself in Volume 36, chapter 342. Nami's breast grew so much, because she was still young and in her growth phase. Especially with the two years timeskip, they had plenty of time to grow!
Reader: Good day, Oda-sensei. You know, that Nami-chan is so gorgeous!!! That beauty, that figure! I'm already swooning ♡ By the way, when you compare her to the Nami-chan in Volume 1, haven't her breasts grown somewhat? by Saori-chan
Oda: Yep. I get this a lot. But when you consider that she's only 18, she's still in her growth phase. She's got more room to grow! I'll give it my best!! (←?) And Robin's only 28! I'll give it my best!! (←?)
After I ended up reading more of the SBS answers, more measurements were revealed. So as we are all curious as to the question how much they have grown, I will additionally post those in the answer, Nami's sizes!
In Volume 6, chapter 49 Nami's Bust/Waist/Hip (BWH) measurements are revealed to be 86/57/86.
Reader: When the question came up about Nami's measurements, you promised to tell us sometime. So when are you going to tell us?!!
Oda: Hehe. I have made a friend that comes in quite handy for this. His eye is quite accurate. It's the "weird chef", who can tell a girl's measurements with just one look, Sanji-san!!
Sanji: Shitty guy! I'm not "weird"!! It's "love", I'm a "cook who loves"!! Oh but, you called me. I get it, leave it to me. Nami-san's cute, isn't she. She also has style to match. Now, let's take a look... B 86! W 57! H 86! No doubt about it!! But she's so cuuuuute. She's definitely a thief. But I still like her!!
Later in Volume 37, chapter 353, which was still before the timeskip, it was revealed that her bust size had grown to 95, making her new BWH measurements 95/55/85.
Reader: Hi, Oda-sensei! In Volume 36 you were saying that Nami's breasts had grown in size from years ago, so what are her 3 sizes now? I'd bet that every man in the country is dying to know this, so don't hold out on us!
Nami: Height 169 B 95 W 55 H 85. Robin: Height 188 B 99 W 59 H 89
Later, in Volume 42, chapter 406, which was again before the timeskip, Oda confirmed that Nami's and Robin's breasts were I cups.
Reader: I am "Sogeking-ko", a proponent of all healthy young boys everywhere! Healthy young boys of this country!! Remember the question in the SBS of Volume 41, asking about Nami-san's cup size?! According to my rotoscopic vision, Nami-san's under bust size is 65. So her regular bust size of 95 would make her an I cup!!! (donnn) That's HUGE!!! I'm a D cup... Boys... can you accept this answer??
Oda: Well, I must say I received a lot of answers to this question from my female readers. Thank you for your detailed and in-depth explanations. But you know, the boys aren't gonna read any of these complicated, difficult answers, so I'm just gonna report on the most common result! Nami and Robin are I cups. Wow.
Finally, in Volume 69, chapter 686, after the timeskip, her new size were revealed to be 98/58/88, J cup. It seems like she had grown a bit more. I wonder when her growth phase will end.
Reader: About the mystery of Nami-san's New World bust size from volume 66, I would like to provide my opinion as an employee at LADIES' INNER. Nami-san's bust size is most likely a "J Cup". There is a possibility that she may have even grown bigger than this. ...If my calculations are correct, her band measurement is 63cm, and her bust is 98cm. If we were to use these measurements, she would end up being a "K"; however, right now in Japan, there is no such thing as a "K" cup. She'd have to order from overseas. Oui, in other words, non-standardized!!! Well, personally, I'd say that Nami-san is a "Nami Cup". (Boiing)
P.N. Shimizu Chageburo's secret fan
(Translator's Note- LADIES' INNER is a Japanese female underwear chain store.)
Oda: Alright. Boys, the day has finally come. This fire that readers started on volume 66. Thank you all for submitting these countless letters. Sanji-kun! Has your nosebleed stopped yet!? Now, give them the presentation!
Sanji: Yeah... sorry about last time. So it's Nami-san and Robin-chan's current 3 sizes, right? They did in fact grow in size, but don't be tricked!! In these 2 years, what really improved is their "sexiness"!
Nami- B: 98 (J Cup), W: 58, H: 88
Robin- B: 100 (J Cup), W: 60, H: 90
Disclaimer: I am no expert on women's sizes and measurements, so if I had made a wrong interpretation somewhere, please forgive me.