Breast size is not a thing I even considered when thinking about who would play Motoko. Or something I would consider tbh. Just because she's done a few Marvel films doesn't make her a great action star.
Anime has a very strict FORM.. The ratio for Chest/Waist/Hip are defining characteristics for --the characters--
YOU personally may not consider it important.. but the majority of anime fans adhere to the original formula..
And since these fans will compose the main audience base.. When you produce the film, you have to consider that..
This is not to say, the fan sits down and measure, but when you are trying for an adaptation, lack of the proper ratio will draw an un-canny response from the viewer.
This is the same reason why Wonder woman and Supergirl looks weird compared to their comic book counterpart..
Their human body ratios are completely off from their cartoon/ comic version..