Zoro's body I see changing a lot, and not just because he gains more muscle constantly. I think his body fat percentage/BF% probably fluctuates, too, and he doesn't track ANY of it, so he can range from eerily cut (low BF%) to unknowingly bulking (raising BF% for muscle gain).

Nami!! I think Nami is the character I have the most variance for in my headcanon for her body type. In my art she ranges anywhere from chubby to "hourglass" to twiggy, but I think if I had to pick one, I see her as having a very straight body type.

Usopp my beloved... I think before the time skip he's a bit of a string bean, and then after the time skip he retains a little bit of the huskiness he picked up on Boin!

Sanji has no ass. The more squats he does the more concave it gets. I will not be taking any questions on this

Aaand Robin. Maybe it's because Robin and Nami are so tied to 2000s trends to me but I see both of them as having the more "straight" body types of the era. Robin, though, is a little more angular, though I wouldn't call her curvy. A very athletic build, for her own protection.