To make it easier for authors to sync their formatting across multiple books or versions, all formatting options including trim size are now saved in your theme. This was a highly requested addition to Atticus for authors who wanted to use exactly the same settings for multiple books.
If you would like to tweak the settings of a theme for a particular book, and not have this change applied to other books, you'll first want to create a duplicate of your theme by clicking the three vertical dots next to the theme title on the Formatting Tab.
If you need more guidance on setting up or editing your theme, I would recommend checking out this tutorial: https://www.atticus.io/create-a-custom-theme/