Oda explained why there are big breasts in One Piece

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 

Oda: “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~!”