Hi,
As far as I can see, it seems to use the thumbnails size instead of the product page image size for the display on your website.
That's not the case by default in HikaShop and of course it uses the correct settings normally.
I can also see that the HTML of the product page has been customized (either by you or your template provider). I'm thinking that the problem comes from that.
Switch to the default template of Joomla temporarily to confirmed whether that's the case.
If yes, they try renaming the file templates/YOUR_TEMPLATE/html/com_hikashop/product/show_block_img.php in order to deactivate it, and it should then display properly.