Product Image Small

The main image is small because you made it small in the Images option area of the configuration of HikaShop.
The thumbnails below should be smaller than it actually but your template is forcing their size.

You will have to add additional CSS to force the thumbnails to be smaller than the main image in order to counter balance your template CSS:
#hikashop_small_image_div .hikashop_child_image{
width: 100px !important;
}

Product page image width

The problem is that you're using the CSS:
#hikashop_small_image_div .hikashop_child_image {
width: 100px !important;
}

in order to force the size of the thumbnails. But you should instead use:
.hikashop_small_image_div .hikashop_child_image {
width: 100px !important;
}

That CSS will also work for the variant images.