hikashop_product_main_image

This is the height of the thumbnail that you configured in the size options of the images/thumbnails in the configuration.
It is necessary in order to display badges properly because on some websites, the images don't have all the same ratio and thus not the same thumbnail height and without forcing the height, badges wouldn't display at the same place for each product and it would be ugly.
If you want to force it to auto, simply add a line of CSS:
.hikashop_product_main_image{ height: auto !important; }