In the HikaShop backend, under -> System -> Configuration -> Main tab -> Images, what do you have there for values "Image width" and "Image height"?
The documentation says:
Image width and Image height : You can specify a maximum width and a maximum height for images attached to products. If the images appears to be bigger, they will be resized automatically during the upload process. If you do not want to resize images, leave these fields blank. These options only apply to the "zoomed" image of products on the product page and to the current category image on category/product listings. Note that if you change these values, the images you already uploaded in your products/categories won't be resized as this process is done at the moment you upload them.
So, no values = original image size in popups, just as uploaded (only limited by screen/window size).
Some values smaller than that = popup size limited to those values.
And all that only for images uploaded since you entered these values.
Wrap-up: Empty those fields. Because I guess that along with those "few changes" you made, you entered some values there that happen to be identical with "Product page image width" and "... height"?