Hi,
In your website, the thumbnails are 200x250, the product image are 400x500 and the full images are maximum 800x1000.
In the product page that Mohamed linked, the full image is 800x800, so HikaShop didn't touch the image to reduce his size (for the full image).
HikaShop can just reduce the image size, not the inverse.
If the image in the popup is not big enough, it means that the image you upload is not big enough.
(and please note that the popup system will try to display the image without going outside of the screen, so there will be a scaling made by your browser if your browser size is too small).
Regards,