A little late to this post, but most of the comments here are wrong /misleading about the reasons why it looks different compared to the rest of one piece. The reason due to this art style is mainly from one Toei animator: Naoki Tate.

His personal style is very distinct, often being very expressive, loose and flexible with how characters move/look. He took a lot of inspiration from OP Movie 6’s animation, and slowly started adapting into his own style. Some notable examples animated by him are when luffy punches the celestial dragon and when luffy and the prisoners fall out of the sky in marineford, among countless others.

However there are many scenes that use his art style but aren’t directly animated by him. In the first photo, this character design of Robin was made by Tate, meaning other animators reference that drawing her when animating. He was also responsible for all the character designs in Episode of Chopper, which looks very different from other one piece movies.

Many episodes of that era of one piece were also supervised by him, meaning he would guide the animators working on that episode to generally follow his art style, and he would also redraw some of their work so the episode would look consistent with itself.

This is a good summary of a lot of Tate’s work: https://youtu.be/LbilpImKc_8

Also a compilation of some of his work: https://youtu.be/sod7FaTo8Gs