Overwrite `vendor\magento\theme-frontend-luma\etc\view.xml`
I have the following, for example: `app\design\frontend\[CustomTheme]\default\etc\view.xml`
<view xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/view.xsd">
<media>
<images module="Magento_Catalog">
<image id="product_page_image_large" type="image"/>
<image id="product_page_image_medium" type="image">
<width>700</width>
<height>420</height>
</image>
<image id="product_page_main_image" type="image">
<width>700</width>
<height>420</height>
</image>
<image id="product_page_main_image_default" type="image">
<width>700</width>
<height>420</height>
</image>
</images>
</media>
</view>
This will cause the `fotorama__stage` to be smaller - it adjusts based on the image size.