Fancybox shows a picture in a smaller size than the original, how to fix it?

Question

JavaScript

I'm using jQuery Fancybox to display larger images. For some reason, when you click on the image, it is shown in a slightly smaller size than its original size, although the width and height of the screen are enough with a margin. The code:

$(".single_image").fancybox({
        'width': '90%',
        'height': '90%',
        helpers   : {
            title  : {type: 'inside'},
            overlay : {locked : true }
        }
    });

What's interesting is the change

'width': '90%',
 'height': '90%',

on any other value has no effect, as if ignored.
How can I make sure that the image is displayed in Fancybox in the original size, if possible?

Answer 1

gregorypetrov original picture has a low resolution?
this is the problem,
add to the src of the picture that will be opened in the lightbox a photo with a higher resolution