In order to decrease the zoom, you need to upload an image of a bigger size. In order to deactivate the showing of the second image, find the option in the Theme Settings > Shop > Product Style > find and disable “Hover image” option
Sorry, I give the wrong instruction. You need to disable “Hover image” in the Theme Settings > shop > Product style. In this case, when you hover the image the first image would be zoomed and the second would not appear. In order to decrease zoom, you need to upload images with the smaller size if you want to remove zoom at all, let me know I will give the custom CSS.