As many people are aware, ladies in anime are often quite... top-heavy. Particularly with the women of One Piece, the busts on these ladies can be staggeringly large, and ultimately disproportionate to the size of the waists on them.
There are always going to be people who cosplay these characters, even those who can't foot the busty bill. No one I know has a double zero waist with a triple-d bust, despite what these anime would have you believe.
One cosplayer has found a way to maintain faithful cosplay adaptations of these characters, while also affording herself a slight more modesty over her chest.
That cosplayer is Nameko Children, a Japanese woman who developed what she calls the 'fluffy bust'. She even did a walkthrough in English to show how she creates these larger than life busts.
As you can see here, the bust effectively sits over the cosplayer's chest, attached to a collar. The bust is then sewn into the cosplay, integrating it in such a way that it appears natural, like in the first photo in this card.
Once she's done the bust and sewn it into the costume, I imagine she would then apply makeup and other kinds of things to make them the right skin color for herself, so it looks like it's actually her own chest.