WooCommerce itself registers support for the default themes. For Twenty Fifteen it registers the following:
add_theme_support( 'wc-product-gallery-zoom' );
add_theme_support( 'wc-product-gallery-lightbox' );
add_theme_support( 'wc-product-gallery-slider' );
add_theme_support( 'woocommerce', array(
'thumbnail_image_width' => 200,
'single_image_width' => 350,
) );
This is why those settings are not in the Customiser.
However, regardless of what theme support is registered, the gallery_thumbnail_image_width size is never editable in the Customiser.
These default settings were chosen for a reason though, so if you need to change them then that suggests you’re changing the theme in some way, in which case you should be working on a child theme, rather than modifying the original theme. In that case your child theme should register its own support for WooCommerce. Then you can either omit the values so that the settings appear in the Customiser, or set your own.