Ok, I figured it out.
Although the hikashop_main_image suggests it is the main image, it is actually a thumbnail.
So to do that I need to change the thumbnail size in the configuration to the size of the big image I want.
Then all the thumbnail images will be displayed big. I then had to use css to limit the size of the images inside hikashop_small_image_div.
So in short it was just a confusion on the div names in the front end and the parameters in the admin.
I hope it helps someone else looking for this.
Cheers