Depending on how condensed the plot is in a manga, the number of episodes that come out annually will be affected. For example, Haikyuu!! had 25 episodes per season for the first two seasons, and since it has almost caught up, there will probably be 10-12 episodes per year from now on each covering 4-5 chapters, as 50 episodes got them to about chapter 150, if they want to stay at pace with the manga.
In an action-based manga where a lot of the scenes are fighting scenes, it becomes pretty hard to lengthen a chapter time-wise. What are specific time-filling techniques One Piece using to keep the chapter-to-episode ratio at 1:1?