If your product thumbnail has empty space, like the one above and you don't want to change the image dimensions, you can choose to apply a fix for this 

In swatch.liquid file 

change background-size:contain; in the following code to background-size: cover;

{% if section.settings.variant_swatch_image == 'variant_product' %}background-image:url({{ variant.image | img_url: '120x120' }});background-size:contain; background-repeat:no-repeat;background-position: center;

                        {% endif %}